Forming a limited company means that your liability is limited. That is, the oners and the company are totally separate legal entitites.
This means that if the company were to fail, the owners would not have financial claims against their personal assets.
In practice, banks often require a debenture or guarantee against a business facility so you are rarely completely in the clear!
If you are the owner it also means that you can extract profit in the form of a dividend and pay capital gains tax on it at 10% (rising to 18% in 2008), as opposed to income tax at 20-40% plus Narional Insurance.