Hardcore, do you want info on the real-life units or the ingame units? Look here for excellent general info about various units:
http://www.fas.org (ok as a starter site, some infos are a bit vague)
Btw, i think what hansundfranz
wants to say is: Just experiment a bit with them in a campaign, certain orders are only accesible on special units-> engineers repair airbases etc...