When a new member reaches 50 posts, they gain 1 power. For every pre-determined post count after that, they get a power. For every 100 posts, you get a power. To give a rep comment, click the little star icon at the bottom left of any post and you can either leave a comment or your name so the person knows who to thank.
The number of powers a person has, is how much they bring up the person's point level. For example if I repped you, you would gain 250 points, and 2, possibly 3 powers.