In addition to all that CD mentioned, I usually, try and write down a bit about the weather conditions (sunny/cloudy/windy/calm), water conditions (calm, surf, etc.), visibility, bottom composition (sand/gravel/silt/etc.) any fish/critters seen, air temp, suface temp, temp at depth, time of day, etc. Boat and charter operator are also marked down when applicable. Site conditions can also be interesting to record, since wrecks deteriorate with time, and sadly, some artifacts may not be there the next time you dive the site.
I'll also mark down how the dive "felt". Anything I was uncomfortable with, or any skills I feel I need to work on.
Anything that I can reference back and either use to remember the dive, or plan a future dive gets recorded. I'll usually write down brief items immediately after the dive (pressures, times, depth, etc.), and then write a more detailed description as well.
Your log book can be a simple or detailed as you like. For my fist several dives, I kept very detailed logs for every dive. More recently, log entries have tended to be more brief.
Your log book can be just a list of your dives, or it can be a tool. Looking back at your old and recent dives can be useful for planning the next dive, or for setting your own individual goals based on your experiences.