Learn how to use Disk Copy or Disk Utility to create an encrypted disk image, which requires a password to open or become available (to "mount"). This document applies to Mac OS X 10.2 or later.

You're at an open wireless hotspot, but you don't want to send your web browsing data over it in plain text. Or you want to visit a non-work-approved web page from the office computer without the IT team finding out.