Although grammatically correct, Vous voudrez une tasse de thé ? will relate to an event in the future, meaning something like, Will you be wanting a cup a tea?

For, Do you want a cup of tea? you will say, Vous voulez une tasse de thé ? (or voulez-vous une...) more than the polite form Voudriez-vous une tasse de thé ? which sounds stilted. But in shops or in the case of any dealings with strangers you will use "je voudrais", not "je veux" which sounds rude in French!

Hi Joan, 

It is the difference between, 

'Do you want a cup of tea? ' and 'Would you like a cup of tea ? '.

The British (being more polite) would always use 'would' but,  'voulez-vous' is acceptable in French to ask if someone wants a drink....