Here's A Couple Of Links That May Help -
You should ask mapping questions in the mapping help thread before people get angry about a new thread being created ;)
The Entities list on The Forge and the QMapspecs are a good start to give you a general idea. However, they are missing certain things. Also a good idea is to open some editor definition file and see what's there (although WC's fgd may be too complicated for that).
Most entities have certain default values that apply automatically if the fields are missing or the value is 0, e.g. the default speed of a door is 100. There are a couple of standard fields that can be used in most entities, like target, targetname, killtarget, delay, wait.
Trenchbroom is currently lacking the functionality to display additional information on the entities and their available fields. Unfortunately, this is only planned for version 1.2 or so.