My "GM Notes" is a single page listing the things I want to do in order that session.
1. Intro new Improved Grab house rule
2. Give out karma points
3. PC wakes up in dragon's lair, negotiate
4. Door closes on other PCs (DC 25) & clatter - skeledemon
5. First PC - escape or accept dragon's quest
6. Party reunite
7. out door after cleric
8. Track - 2 miles - soft ground, then hard, if lose trail "grig footsteps everywhere -- dance last night"
9. Ranger's companion circles overhead (Nature DC 15)
10. Return quest reward - if to Lady Arabella - "Want to be sheriff?" if to Sheriff Monte - reward potions CMW CMW and Arabella pouts
As you can see, I'm a big ol' railroader. Although there's a little bit of Justin Alexander's "situations not plots"
timeline idea in there too. And in actual practice there was a showdown with the dragon in the middle of step 8 that isn't on the outline because I didn't plan for them to betray him.
The other half of my notes is from during the session -- stuff I didn't plan for and need to remember. I also mark down what time we got through with encounters just so I can evaluate my pacing. (this is from a different session)
hairpin is red coral-shell
11:30 PM in rat tunnels
ogre: shot in plumber's crack
PC uses Sleight of Hand to switch religious symbols
dwarf haunts the well
bard promises wand CLW if his girlfriend gets the hairpin
Sharra & Arlen date: drop anchor in rented boat
3:36 AM Fight with assassin over
All that stuff like campaign history, rules I look up a lot, etc, I don't think of as GM notes -- they're "reference." One place where this blurs is NPCs. I keep each NPC on his own note card and I have his history and attitudes on there, but I also add things to his card that develop during play, like who gave him the finger.