For a decent article on this, go here: http://www.fulltiltpoker.com/pro-tips-archive.php
#; click on All Pro Tips and scroll down to #172: Taking Online Notes on Opponents by Paul Wasicka.
On FT, I try to color tag everyone as an initial "note". Orange = Lunatic; Blue = Passive; Red = Aggressive; Green = Tight. Lavender = known password thief/sharer/begger. Unfortunately, this nice little function isn't available on other sites, so you have to go with just notes.
Then, as the game progresses, I try to add: What (game; NLHE, LHE, Razz,, StudHi, Stud8, PLO, etc.); When; Style (AIM [All In Moron]; LAG; TAG; Weak; Passive, etc.); particular patterns like aggressive bets primarily when in CO, Button, SB, BB and the like. In hands that go to showdown, I like to use the Replay/Last Hand button to check on what they were willing to call or raise with. When my Pokertracker 3 was functional, I frequently added data from that to the notes for future reference.
Poker is a game of information and the more you have, the better off you are.