Edit: after I posted what follows below, I realized that Daniel had asked what pokertracker was. Short answer: it's a program you can use to track your online play at most of the biggest online sites. It gives you statistics about your play and your opponents' play as well: for example, it can tell you how often you voluntarily put money in the pot (vs blind money of course) -- which can help you see how loose/tight you are playing. It can also tell you the same about an opponent you have been playing against (but only for those hands you have in your database), which can be very valuable information. It's $55, but you can use it for 1000 hands on a free trial. The website is
http://www.pokertracker.com/. (OK, maybe not so short!)
I've not used PT because I'm new to online play, but the 2+2ers swear by it. Indeed, there are some great threads about PT over at 2+2, some of which I've bookmarked.
One classic is bisonbison's notes on notes, which also has a link to a couple of MS Sunshine's posts (both are veteran, well-respected 2+2ers). The link:
http://forumserver.twoplustwo.com/sh...t=1&PHPSESSID=
Many of the 2+2 forums have FAQs that have good links as well; the microlimit FAQ has a brief overview of desirable PT stats, along with a warning that a sample size of less than 10K hands is probably not enough for meaningful stats.
Indeed, the need for a large sample size is one reason why I never bought PT: I never thought I would play online that much. But I've gotten sucked in (darn CT tournaments!) and so will probably join the PT devotees soon.
Hope this is something helpful to start with until the experienced chiptalk pokertrackers chime in!