Diamonds are pretty standard currency: posting new topics and replies in forums will award members with Diamonds, which can be traded for artwork or services. Posting a new topic will give 2 Diamonds, and replying will give 1. You can use these between members for shops and other various things.Subject to change per upgrade.
Kisses were phased out of existence Kisses January 12, 2011 for 250 diamonds per Kiss. There were acquired for outstanding behavior, for being helpful, for participation in certain events, for improvement in dolls, and a myriad of other fun reasons. Essentially, they were meant to encourage good members, not just ones who post a lot. They were unable to be used after the upgrade but may reappear at a later time.