[Unit help?]

I saw that there are two types of engineering classes : Technician and Engineer. Technician has a gun while the engineer has a repair arm with a laptop. Can you please explain what these do?

[Unit help?]

So I just started playing tiberian sun reborn. I plan on playing multiplayer soon but I have no clue what the units do or anything. I cant even get the technician class to repair a tank in skirmish! Can I please get a list or a link that details each unit? Thank you.