When you play tabletop roleplaying games, whether you're the player or the gamemaster, keeping notes helps remembering, especially if you've forgotten something completely, in which case written notes can tell you black on white what happened, unless your notes are crap like some of mine.
Past note keeping
In the past I didn't really keep notes. I kept adventures, written in note form, characters, and props. Years later I tried to get an overview of my home-brew world only to discover that my lack of keeping notes meant that I had a very limited idea of what actually happened so many years ago. My favorite note is "scary doctors" . There was no more or less on my piece of paper and to this day I have no idea what it means.
Current note keeping
Over the years I learned to write adventures in a more professional manner, which helps a lot in keeping track of what has happened during sessions. Furthermore I've also learned to write notes during adventures. I've tried to sometimes let other players take notes, but that sometimes poses an issue when that player forgets to take notes, takes the wrong notes, or simply isn't present.
One way I keep track of everything, and one way I've suggested in keeping notes when playing games with multiple gamemasters, is to use a 3rd party program such as Obsidian Portal. This is an easy to use website that works a bit like a wiki with all the information you want to keep.