The only thing is you own your domain name until there's another company that wants it, in the case of the UK's domain name extension .co.uk they will have to take you through a Nominet dispute resolution process, and if you don't have deeper pockets than the claimant or don't know your stuff, in most cases your domain name will just transfer to the other company!

This is all to do with cyber-squatting etc but larger companies exploit the rules to their advantage and will 'legally' steal your domain name! For want of a better explanation! Thats called Reverse Domain name hijacking! (Wont go into that quite complicated!)

But until any silly stuff like that takes place it's all yours until you dont renew the annual fees! So if your domain name is important to you ensure you keep the contact details updated with your domain registrar, so when it comes to renewal time you receive their email and act on it! If a greedy cyber squatter grabs it he/she will basically hold you to ransom to buy it back! Dependant on the quality of the name, backlinks, page ranking, and traffic!

But then you would have a legit right to conduct a Nominet dispute resolution process yourself and take the name back through legal means!

Hope this helps!!