I have played 45 games of Online Chess and my RD is 109.  I just finished a game (actually yesterday so I am sure the stats are updated), but my RD didn't change at all.  How so?

This is probably a good place to start:  http://www.chess.com/article/view/chess-ratings---how-they-work

If you make it to the bottom, there are a couple of 'further reading' links, one specifically on the Glicko system.

If you take a long time between games, the RD goes back up.  The idea is to get an 'accurate' indication of your strength.  So, if you play one game in March, then study a ton, and play another in June, for instance, you could very well be 100 or even 200 pts stronger in your play.