Do people give you a karma once you help them, or do they give you a karma just to be giving you one?
karma can be given to anyone by anyone. if you agree with someones post you can give them karma. if someone has been helpful to you, give karma. karma can be given for any reason.
Mostly the idea is, if someone is helpful to you or kind to you, or has really gone out of their way for you, etc, etc, you can give them a karma point. Realistically, karma doesn't mean much - your TR rating is much more relevant. But karma is still nice.
You give karma by clicking the +karma link. You can only give karma once every 6 hours.
If you'd like to give me karma for answering your question, it would be appriciated (but not required)
I'm definitely giving you a karma.