yaw = Current Model Yaw Position which is the left/right angle (Y angle)
ideal yaw = Desired Yaw position (Not used by engine but used by a Bot DLL for example to slowly change actual yaw towards desired yaw)
pitch = Current Model Pitch Position which is the up/down angle (X angle)
ideal pitch = Desired Pitch position (Again not used by the engine but used by the user program to smoothly change the pitch towards the desired pitch)
see:
http://wiki.bots-united.com/index.php/Player_angles (Login
user: wiki,
pass: nospam
maths/physics articles for games:
http://www.gamedev.net/reference/list.asp?categoryid=28