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 account will maintain its own custom content and configuration files.
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 using one Steam account simultaneously - 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).