Karma is the Sanskrit word for action. It is equivalent to Newton's law in physics, that every action must have a reaction. Every though you think, every word you speak or act you initiate, initiates a force that will react accordingly. This returning force maybe modified, changed or suspended, but most people are not able eradicate it.
This law of cause and effect is not reward, punishment, payback, or retribution, it simply is how things function in the manifested universe.
A person may not escape the consequences of their actions, and will suffer only if the person himself or herself has ripened the conditions for their suffering. Ignorance of the law is no excuse whether laws are man-made or universal. To become empowered in the worlds of karma and reincarnation here are some pointers about karmic laws.