When playing casino poker, we see our opponents. I seldom forget a truly good or a truly horrible player. Online, all we see are little icons, many with similar or extremely non-descript names. Plus, we play against thousands of players from literally all over the world, playing at all different hours of the day or night -- all at the same time! Player notes offer a way to remember notable players. For me "notable" means the best players -- and the worst goofballs. For the most part, the vast majority of players are interchangeable weak-tight lemmings. You don't really need to keep track of them. But strong players and terrible players come in much smaller numbers. Also, since I play mostly Omaha8, I keep track of super-tight players. These players aren’t "strong" players, and may not even win, but when one bets an 876 flop, I know to put my A3 in the muck. Depending on the game you play, you might also want to keep track of other player categories, but I definitely suggest you track the best and the worst. Most cardrooms now make this easy by including a "Notes" feature where just by clicking on a player's icon you are able to then make notes about that player that are permanently stored on your computer.
Hand Histories
By clicking the dealer tray, you can have your "hand histories" sent you, the records for the individual hands of the games you played in. Novice players can find these valuable learning tools, and even experienced players can use them to study the play of specific opponents. Commercial programs are even available that can analyze tens of thousands of hand histories in a flash and spit out a wide variety of statistics on you and your opponents.
Net Stats and All-Ins
The Network Status page is your friend that you should regularly check when you play. The NetStats tell you how the cardroom server is performing. Beware of playing whenever the status is below "Good." Not only do you risk being disconnected in critical situations, your game will be painfully slow and naughty players will use the server problems to illegitimately go "all-in" in situations where they merely don’t want to call any more bets.
When legitimate, an all-in occurs (besides the obvious when a player has no more chips) when a player loses his or her connection to the game, either because of being bumped offline by a flaky Internet connection or because the cardroom is having server problems. Sometimes you'll lose pots due to legitimate disconnects, and other times you'll win, so you shouldn’t worry about them that way. But if you suspect someone illegitimately times out because they don’t want to call a bet (or two or three), contact Support. Such a person may get away with it once or twice, but eventually they will be stopped.