Karma is a Hindu concept that means "act, action, or performance". It is understood as that which causes the entire cycle of cause and effect in Hindu, Jain, Sikh, and Buddhist philosophies. Though the explanation of karma differs from tradition to tradition, the general concept is the same.
Through the law of karma, the effects of all deeds actively create past, present, and future experiences, thus making one responsible for one's own life, and the pain and joy it brings to him/her and others (source: en.wikipedia.org/wiki/Karma).
Pagans and Wiccans have their own views of karma. Wiccans believe that everything you send out comes back to you times three. However, many Pagans and Wiccans only apply this law to people who have upset them.
When they feel wronged, slighted, or persecuted, some just sniff and say,"Well, that will come back on so-and-so times three!" and perhaps they add a "So Mote it Be!" just to make darn sure.
Is Karma a bitch? Yeah, kinda. Can we control it? Not so much. If you do shitty things to people, do shitty things happen to you? Yes, but that doesn't necessarily mean it's karma. No one knows how long it takes for us to get what's coming to us, but we do, eventually. Don't think you control it, babies. You can't decide who gets what, or when. If you try to control the universe, rest assured it will blow up in your face.
Sit down and hold on tight, kids. It's a bumpy ride.