Steam games reside inside the steamapps folder in the Steam directory of Program Files, a generally shared location on a Windows PC.
If you install the games on a computer, any user on the computer has access so long as they log in to an account that owns the respective games.
As far as another PC is concerned, some steam games have a backup feature available in the Library GUI of the Steam application that will make transferring easier. It doesn't work for everything though, and sometimes just copying the proper steamapps folders/contents to the same place on the new PC will do the trick.
As far as transfer medium... USB or Local Network are your best bets.