Player notes are such an invaluable tool that it's worth a short page all about them.
Okay so lets get this straight, you want to play online poker and win a little money?
Right so to be able to out play the people you meet you will need to get to know how they play, you need to work out their vulnerabilities, and you need to exploit them.
To be able to do this with any success, you have to make notes, you will come across thousands of players online, and chances are high that you will play against a handful these more than once. This means that if you keep player notes, you are at an advantage as soon as you sit at a table, everyone else there (if they don't make notes) will need to watch you play for a while before being able to work out what type of player you are. You will be sitting down with your notes, and you will instantly be able to see what type of players you are up against. (because you made notes on them right?) You may be able to call a bluff from someone, who has been on the table for a while, who thinks they can steal your blinds till you get settled at the table. You know this as they do it to everyone who sits down at the table with them, and you made a note of it last time didn't you?
How to make notes
Most poker rooms offer a player notes facility, so notes are available automatically for all the players at the same table as you.
If you play at more than one room online, you may want to cross reference players, as there is a good chance that they will use the same screen name at more than one poker room.
First identify what type of player they are, Loose or Tight, this can be one of the single most useful factors in the decision making process, when playing against them.
Second identify whether they are aggressive or passive, this isn't so easy to work out, but people who like to go all-in you can put as aggressive. People, who will bet all the way to the river and show a flush/straight draw, are aggressive. Someone who has top pair top kicker and only checks or calls is a passive player.
Take notes on whether a player will go on tilt if they are almost busted out, or if they like to play short stacked, and buy-in with another short stack, when they are busted.
Does a player bet out on a straight or flush draw to knock out weaker hands, or will they check it and hope to get a free card and hit their draw.
Try to keep your notes concise, use abbreviations if that helps.
Don't make notes when you are angry at being busted out or after a bad beat, they won't help you in the long run
Be consistent with your notes, try to set a format that you can use for all players, and that you can scan quickly
Remember that professionals have been keeping notes on players in real poker rooms for years. It's a technique that works - the sooner you start us