OS X is designed for multiple users of the same computer.
Each user can have his/her own login account with or without admin privileges. Each computer login account has its own Home folder/directory which is kept separate and secure from other users. Automatic login can be enabled or disabled and when disabled, each user login account must enter his/her password to login in to their account. You can enable Fast User Switching to make the process of switching between multiple computer login accounts a fast and easy process.
With each login account having a separate Home folder/directory, each login account can access their email account or accounts only with the Mail.app, their contacts only with the Address Book, their calendars and calendar events only with iCal, their photos only with iPhoto, their music only with iTunes, their documents only in the Documents folder, etc., etc., when logged in to their user/login account. When syncing their iPhone with iTunes when logged in to their user account, they are syncing their data only.
Each user/login account has a Shared folder for sharing a document with other user login accounts.
