What happens if I change SIM cards on an iPhone 5?

I have an active iPhone 5 on AT&T. If I put my friends active iPhone 5 SIM card into my iPhone 5, what happens? Will I lose my contacts? Will it restore my phone? Will it change my phone to his plan? Or will it just show different contacts on the phone instead of the ones on my SIM card?

Added (1). My friends SIM card is active on AT&T. Will it just replace my phone contacts with his/

Added (2). And will it change the phone number to the new sim card?

You haven't said whether the phone is locked to At&T or not. In either case, if you add your friends sim card,
You will lose only the contacts saved in sim card. Contacts saved in phone are not lost.
It won't restore your phone. Your phone won't be changed to your friends plan. For your last question, yes. It will show the contacts stored in your friends sim card + contacts stored in your phone.

Contacts? No
Restore? No
Plan? Temporarily (as long as it is in the phone)

Again, contacts are not affected since they are stored on the phone.
Yes the number would change as that is on the SIM