http://www.chess.com/forum/view/help-support/how-do-i-get-points
That should help you.
Member Points: Chess.com keeps track of all of your contributions to the site such as comments, blog posts, forum participation, articles, etc. Each contribution counts for points. This helps members know how involved each other are in Chess.com!
some are worth more than others. the kinds of contributions that are more meaningful (articles, etc) are worth more.
at 1,000 points i get you a free t-shirt (as long as they are not achieved through spammy methods)
UPDATE => Nov 11 2008 - we do NOT give out free shirts anymore (we did this for a few weeks and stopped in 2007).
How do you earn member points and what are they for?