Karma Sanskrit; Kamma Pali:

When we speak of karma, what we are referring to is the the Buddhas teaching on the law of causality, or that of cause and effect. Karma is a dynamic process of inter-related causes and effects, actions and results, meaning that it is actively functioning in every moment and in all things which are within the realms of existence. All things reap and inherit the effects of karma. To begin to understand karma we should know that all actions stem from a cause and come to results, which in turn are a further cause for further results and so on ….. this being the case karma is a active and reactive turning of events and their effects, all things being inter-connected within its workings.
The law of causality is a law which can not be broken, spoken, stamped on paper, or effectively corrupted, it is simply the law which is always working behind the scenes in all our actions and the reactions which occur throughout our experiences in the inconceivable amount of re-births we have undergone in the cyclic existence of birth and death, causing us to be re-born and go through living and dying over and over, being victim to the dissatisfactoriness of life in Samsara*, it is as if we have been wandering lost for aeons, not knowing who we are, searching for our true self in all our experiences throughout time, since time beyond an inconceivable beginning. In our wanderings we have become lost because we have become victim to our senses thinking that we should seek happiness in material goods and wealth, we become a slave to our senses continuing to seek to satisfy our desires, and in doing so we have become entangled in the workings of Samsara* losing sight of who we really are.

Karma works on many levels we could say, it has both subtle and grosser qualities taking effect in an infinite number of ways, yet all results stem from a cause, and all actions belong to the doer of the actions. We each become subject to the results of our own doings, but not necessarily straight away, and not necessarily will we recognise the effects of our actions as being likened to the cause, yet we can know by our own experiences that where we apply ourselves with good intentions we are likely to get good results, and where we apply ourselves with intentions which are motivated by ill-will, greed, selfishness, or hatred, then we will likely get results which we consider to be negative results, leading to further dissatisfaction and suffering. This is because karma is interactive throughout all things at all times and so the effects of our actions will also be influenced and balanced with an uncountable number of other inter-woven causes and effects. But time has proven to us through demonstration that on a larger scale we ourselves are the benefactors of our karma, we can expect to benefit if we act accordingly to the way which is right, and we can expect to experience suffering if we choose to act in ways which are conducive to harm.
To really understand why the simple function of karma seems to work in such a complicated and complex manner we need to further understand that Buddhists believe we have experienced other lifetimes within the realms of existence prior to this one, and whatever residue of karma we leave behind at the time of dying is what the next physical manifestation of being taking re-birth will inherit, and so our karma is not only constantly interacting with all things it is also acting in all times. Over an inconceivable amount of time we believe we have taken re-birth once again in the human realm where we can develop and cultivate our higher wisdom and reap the benefits of doing so, with good results we then should dedicate the merits created by our good deeds to all beings without exception, there is greater merit in doing good deeds for others and cultivating good qualities with good will towards others, than there is doing good things for oneself and close associations alone. The Buddha teaches that we should express ourselves in ways which are wise, and incorporate compassion and kindness towards others, this is also being a cause for merits in the world and is praised by all beings up to and including the Buddhas. We should rejoice at the well being of others, and try to maintain our practice with the effect that we cause some goodness in the world.
It could be noted that there is no good karma or bad karma as such, karma is simply the working law of cause and effect, so whatever karma takes a result it is simply a result, whether that is good or bad is distinguished by our personal preferences and opinions at the time, as such we give it a category in either ‘good’ or ‘bad’, but in reality karma is neither good or bad, it is just the way things turn out according to the deeds we do, so we should take some care in choosing how we act in this life, without due care we are likely to revolve in the cyclic existence of Samsara* getting lost again and again, inhabiting the realms of others where suffering takes on a new meaning, and all because we lived in ignorance. By choosing to develop mindfulness in our ways we can create better circumstances both for ourselves and in turn for all others also, but all this should be done while developing a selfless attitude in life with intentions which are good.
Because karma is a complex system of dynamicly inter-related actions and their effects it is impossible to predetermine how the results of our deeds will take effect, for this reason it is considered to be an ‘unconjecturable’, if one were to attempt to know all the results and causes of their experiences they would still never know even after an inconceivable amount of time it would be like attempting to know the square sum of infinity. It is much better for one to pay attention and be heedful of their thoughts, speech, and actions in order to create causes for positive change, one should know how karma works to some extent, such knowledge is beneficial at times, but it is much more important to be aware of what is going on in the here and now. It is important to know that our karma is not fixed or definite, and so if we have created circumstances which are harmful or cause us suffering then we can still put the teachings of the eightfold path to practice and begin to change things for the better in the longer term. Likewise one should be careful not to undo any good results by being careless in their ways any further than one already has been in the past.

In the Maha-Kammavibhanga Sutta, which can be found in the tipitaka,.. majjhima nikaya,.. the Buddha explains how we revolve in the cycle of birth and death undergoing lifetimes in different realms according to the deeds which we have done and are known for. This means that karma is inescapable because we always know what we have done, and so we are born and die according to our past deeds and our present state at the time of death, we believe we have existed in this way over and over since time immemorial (but not as a permanent entity), it is fortunate that we have encountered the Buddha Dharma in this life where we can learn and benefit from such profound teachings, so hard to come by in other realms and times, and we can begin the way to find our true nature, that primordial essence which underlies all of lifes experiences .
In this sutta the Buddha explains that karma does not always ripen in this same lifetime and so we experience re-birth in another realm, maybe the realm of the heavens, or the realms of the hells, or maybe in the realms of worldlings again, or maybe in another realm, but eventually our karma will take effect and so we begin the process of cyclic life in Samsara again, to understand how karma works in association with re-birth one should come to learn and know the teachings on dependant origination. Maybe we might have done good deeds but these good deeds have not yet ripened to take fruit as good results, we might die and not experience happiness, wondering why we did not experience the results we were sure we deserve. At some time the results of karma will ripen and in re-birth we will be re-born according to our disposition at that such a time. Likewise the same can be said of bad deeds or deeds done with wrong intentions (and their results).
In this way there appears to be karma which is not capable of good results, and does not produce good results ; there is karma which is not capable of good results, yet appears to have good results ; there is karma which is capable of good results, and does produce good results ; there is karma which is capable of good results and does not appear to produce good results ;
So we can see karma is dynamic and is not fixed or definite so we are not under the whim of ‘fate’ as such ; neither is it immediate, the results of karma can become quite complexingly influenced by other karmas and take some time for it to take effect, even over several lifetimes, although it can take effect rather immediately also , so it is completely unpredictable how or when we will reap the consequences of our actions.

The Buddha teaches that the only way to unfetter oneself from the bonds of worldly existence and the workings of karma is to go beyond the state of dualistic seeing and knowing, by knowing how we come into existence and how our sufferings are caused through our own ignorance of the truth of the way things are, by going beyond the notion that there is separateness in the world and within our very own existence, by losing the attatchment to the elusive 'Ego'. Through practices involving contemplation, insight cultivation, metta (loving kindness) meditation, and attaining the consciousness levels known as the jhanas and entering a state of samadhi *, concentration involving the aspect of joy, rapture, tranquility and equanamity, one cultivates wisdom and compassion for others knowing how all beings suffer, and how all beings seek happiness.
One works with the intention of knowing Nirvana or Nibbana , a state of perfection, immaculate and without flaw, void of notions of self and other where things are just as they are, unconditioned and free of marks with which one can identify and distinguish with.

Nirvana (Nibbana) is known, in Buddhism, to be the ‘opposite side of the coin’ so to speak, to Samsara * , Where Samsara is deemed to be the cyclic realms of limited and impermanent existence which is a cause for suffering, … Nirvana can be deemed to be that state which is free of conditions, and where one is no longer bound and fettered by conditions seeking happiness where it can not be found, or desiring things which can not bring permanent satisfaction to ones life, rather one is said to be ‘unfettered’, or ‘unbound’.

Sources:
E-Sangha Buddhism Portal