Currently Being ModeratedJan 21, 2013 4:12 PM (in response to MrsMikeyB)
Time Machine makes a complete copy of your Mac on the first back up. After that, it backs up any changes that have occurred on the Mac since the last backup.
The idea behind Time Machine is that if the hard drive in your Mac fails, you have a complete up to date copy of the Mac that you can restore back to the Mac.
When you have some time, please read through this Apple support document on the Time Capsule and Time Machine for a basic understanding of the process.