I've heard people ask about what should be in brew-logs a few times. The answer really depends on your situation. The one commonality is the goal of a brew-log, which is to provide enough information to be able to recreate your brew, exactly. I try to record everything that I can control. When I was an extract brewer I didn't have to write very much. When I moved to all grain I had to redesign my logs to accommodate A LOT more information.
These are the variables that I record on brew day:
* Recipe * Malt Bill * Hopping Schedule * Yeast * Misc Ingredients * Adjunct Sugars * Spices * Salts * Finings (Irish Moss, Whirlflock) * Recipe Stats * Target O.G. * IBU * SRM * Target Mash Temp. * Procedure Variables * Strike Vol. * Strike Temp. * Dough-in Temp. * Mash Temp. * Mash Duration * Mash End Temp. * First Runnings Vol. * First Runnings Gravity * Sparge Vol. (into mash tun) * Sparge Temp (in mash tun) * Sparge Collection (batch sparge runnings out of mash tun) * Sparge Gravity * Pre-boil Vol. * Pre-boil Gravity * Post-boil Vol. * Post-boil Gravity (O.G.)
Note: The first two sections are important for all brewing setups. The last section is specific to my setup which is a three vessel batch-sparge all-grain system.
Beyond brew day there are other things to note:
* Fermentation Temp. * Fermentation Duration * Additional Finings * Transfers (secondary, bottling, keging) * Final Gravity * Tasting Notes
I also tend to fill all the margins with miscellaneous notes like efficiencies, calculations, and deviations from my standard procedure.
When you keep detailed brew-logs you will be able to recreate recipes much easier. In addition it makes tracking or troubleshooting changes to your system much easier. Of course you won't need to keep logs that are exactly like mine but this is a good starting place. Use the variables that you measure and record them every time for consistent quality brewing.