You can have multiple users or access multiple accounts on the same computer, just click "Sign Out" to log yourself out at the end of each session. You can then log in with a different Yahoo! ID and password.

Please Note: You can only be signed in to one Yahoo! account at any given time on the same web browser. But it is possible for multiple accounts to be signed in simultaneously on the same computer using different browsers (for example, johndoe on Firefox; janedoe on Internet Explorer).