You get points for participating in things like the KatKwiz and just posting. If you'd like you can put points into your bank for safekeeping (if you click the little arrow next to Shop Menu in the little bar at the top you can get to it there). The more you put in your bank account, the more your weekly interest it!
Points are used for buying gifts for other members that appear under their points count in every post you make. Again, if you click the Shop Menu arrow it'll have an option that says Give Gifts - this'll bring you to a page that you can choose a gift and type in the member's name you want to give it to.
Edit: katachtig beat me to it!