Can I use multiple Steam accounts on one computer?

Yes, you can access different Steam accounts from one computer, however, you may only access one account at a time.

The accounts will share the core files for any games the accounts have in common, but each user's game configurations and any custom content will have to be copied across the users' directories if it is to be shared.

Can I use my Steam account on other computers?

You may use your Steam account on any machine which can connect to the Steam network - Steam allows you to download and install any games registered to your account as soon as you log in.

Can multiple users play simultaneously on one Steam account?

Steam does not support multiple players to use one Steam account - games associated with a Steam account are licensed for the sole use of the account holder.

If multiple users attempt to log in and play simultaneously with one Steam account, the user who logged in first will eventually receive an "Invalid Steam UserID Ticket" error message and be forced to log back in to continue playing (if the first user logs back in, the second user will receive this error message).

Do I need a separate account to run a dedicated server?

If you are using a single Steam account for your Stand-Alone Dedicated Server and your game client, you will run into the "Invalid Steam UserID Ticket" error which is presented to multiple users attempting to play simultaneously - if you intend to use a computer as a dedicated server, you must create a separate Steam account to run the dedicated server software.

Note: Client Listen Servers use the Steam UserID ticket for the client on which they are run - the "Invalid Steam UserID Ticket" error will not be presented when you are running a Client Listen Server unless another user logs into your account and starts a game.