Karma is a concept common in Eastern religions and culture, but is interpreted very differently here in the West by many people. Our understanding of karma is largely based on the Hindu and Buddhist concepts of karma, so these are the concepts I’ll focus on here. When most people think of karma, they tend to think of something like this:
But in reality, the concept of karma has nothing to do with this sort of thing in its actual context. People like to think that karma means you “get what you deserve” in this life through some force in the cosmos, possibly because it suits their desire for vengeance without them actually claiming their vengeance for themselves, but that’s not what karma is.
In the West we tend to think of karma as the invisible moral force of reward and punishment, but in the Hindu context karma refers to the action or deeds of a person. In fact, the word karma literally means “action”, “deed”, or “work”, referring to the actions or deeds of a person, and any reward or punishment would thus refer to the result of said actions. However, karma is tied to the concept of reincarnation in which the soul enters a new physical form after death, which means your actions in this life are more or less tied to the next life. In this sense, karma in Hinduism tends to play out more like this:
In addition, Hindu belief stresses that there is only one Self, but instead of the individuated self there’s a single consciousness or Self tying all life together. This means all life is connected, and every individual is not only part of all others but also part of the divine consciousness of the universe, in fact but a piece of this consciousness, thus the goal of Hindu spirituality is for each piece to reunite with this single underlying consciousness, often identified as either God or by the concept of Atman (this itself tends to depend on what school of Hindu thought you answer to). It also means that it’s not you being reborn in a new body, but rather Atman. It also means that the bad things can happen to other people as a result of one person’s karma, which is horrible. In fact, this was one of the ways that people tried to explain the tsunami that happened in Southeast Asia near the end of 2004, and it may sound glib but it’s not an unreasonable interpretation of Hindu belief. Either way, I feel that since your consciousness is not your own in Hindu belief and since reincarnation entails someone or something else inheriting the fruits of your past actions, someone or something else is going to get screwed over by your actions, which is just illogical and wrong.
It works much the same in the Buddhist faith, except that in Buddhism there is neither a single divine consciousness nor individuated consciousness. Reincarnation is still based on karma or actions, but karma was also defined by Siddhartha Gautama as intention itself. This means that even mental action, the thought or impulse to perform physical and verbal actions and that influences such actions, can affect the next life. In broad terms, it also means that even a good action motivated by personal desire can be impure and lead to an impure rebirth, and the highest states of being, along with enlightenment itself, hinge on selflessness, which is stupid.

In general, karma just means actions, but it’s funny how even though the concept of karma entails actions rather than a justice system designed to reward or punish them, the theme surrounding the concept of karma in many religions seems to be that your actions will be punished after this life rather than within, and I can’t seem to agree with the idea that the cosmos dispenses any sort of reward or punishment for actions that, if reincarnation is to be believed, you are no longer technically responsible for. And the thing is, if Hinduism and Buddhism are to be believed, you either don’t have an individuated consciousness, or your consciousness is not your own anyway, so how the hell are you responsible for actions that aren’t even yours either way?