if you mean by someone@something is someones email. then something is the domain name from the server. Someone is the user nick on the server.
you can change uses on a server by typeing su nick. then you switch to user nick. you must type his password before you are nick. to go to root you can simply type su or su root. if you are root you can change to anyone without typing a password.

i hope this is the information you are searching for