Karma is a basic belief, not a religion. There are many visions, but the most common and basic form is that what goes around, will inevitably come back around. You do harm, it comes back to haunt you. You do good and rewards follow. There is however one thing that not too many people realise: Karma, real or not, is only 100% effective if EVERYONE, that is, the entire living population, believes and acts overtly upon it. Negative karma effects EVERYONE like a ripple. Once effected, you will effect others. Our actions effects others so much more than you think.
Somehow, you screaming at your lover, your parents, your siblings and children in the morning, will travel to another place, in another form. Your child goes to school in a bad mood, and causes trouble, ruining a teacher's day. This teacher goes home to their unstable relationship, blows up at their partner. Their partner goes to work the next day and has had enough with 'the new guy'. The new guy is laid off. This new guy posts an offensive blog that angers another internet user in china. 
Although a stretch, and a seemingly tangent-like ramble, in the information age of technology, our actions can take on a global scale, even if as minute as someone having a bad day. 
Achieve peace. It is the only thing that will save us.