Every time you play poker, it's always very important to try and keep an eye on your opponents in order to understand what style they play and what they're capable of. Especially in Texas Hold'em, it is imperative that you know how to play from your opponent and not from your own cards. Thus, you should always try to notice the characteristics of your opponents' game, which will help you in the future in making the most profitable decisions against them. The easiest and most effective way to remember your opponent's playing style in some key spots is to take notes (notes) based on your previous experience with him. Personally, I only keep such statistics on card chat players from platinum level.