The backup copy is stored as a disk image that you can encrypt if you don't want anyone else to have access to it.
The disk image can be dynamic (read and write) or static (read only). In the case of the latter, you'll need double the disk space if you're working with individual files.
It is, of course, possible to compress the disk image and segment it, providing maximum megabyte values.
You can also save a backup copy and give it an expiration date.
For Mac OS X 10.4 and later, including Leopard. Users of Mac OS X Panther (10.3) and Mac OS X Jaguar (10.2) should download Carbon Copy Cloner.