Lucas, Simon's description is spot on.
Finance companies often use Caveat's.
For example, you might borrow money to buy a car and they want to protect themselves more than just a lien over the car, but you don't want a mortgage over your home just for a car loan.
They can lodge a Caveat with the Titles Office, which means you cannot sell it, mortgage it to any one else, without them knowing and without their permission.
If you default on the car loan, it doesn't give them the right to just sell up the home to recoup the car debt, but it does give them some control over what you do with the property - and therefore a better chance of eventually getting their money back.
Geekay