think the difference is in nuance - 'lots of' is for judging the amount - a high amount is good/enough for something, eg: I have lots of friends (that's good - don't worry about me), There are lots of things for us to study (don't worry, we won't be bored), or That shop has lots to choose from (it's good - many options for you) 'A lot' is just a high amount, like many or much: 'I have a lot of work to do' or 'I have a lot of friends/stress' - no comment about good or bad. ('I have lots of stress/work to do' seems not as natural) :) But this difference is very small - most people use it this way and don't realise it.!

Some comments on the previous answers: I agree with Hannah about the question of formality. Saying 'lots of' sounds rather more informal than 'a lot of'. Depending on the context, 'lots of' can sound more enthusiastic, or slightly more childish than 'a lot of' - hence the greater degree of informality. Please don't be misled into thinking that 'a lot of' is formal, though. In fact, both are informal expressions. If you're writing a business letter or an essay, you should avoid both 'lots of' and 'a lot of', and replace them with a more formal expression such as 'a large amount of' ( for uncountable nouns) or 'many/a large number of' (for countable nouns). Keith has also made an excellent point, and it's something that you rarely see in the grammar books. While 'a lot of' and 'lots of' are sometimes just synonyms, 'lots of' can also be used to emphasise the idea of 'plenty' - in other words, an adequate amount/number. For example: A: Should I call a taxi to take us to the station? B: No, we've got lots of time. Let's walk. A: But we have got a lot of luggage. B: You're right. Maybe we should get a taxi. B says 'We've got lots of time' to mean 'Don't worry, we have plenty of time'. This is used to refute the assumption that they might be short of time. A native speaker would probably not use 'a lot of' here. The function of the statement 'We've got lots of time' is the opposite of that of the statement where A says 'We have got a lot of luggage'. Speaker B is saying that there ISN'T a problem when they say 'We've got lots of time', while speaker A is pointing out a problem by saying 'We have got a lot of luggage'.

I believe the only difference is formality. "lots of" is more informal and can almost be considered slang and "a lot of" is more formal and proper. Examples: I have lots of homework to do. I have a lot of homework to do. I hope you receive lots of gifts on your birthday. I hope you receive a lot of gifts on your birthday.

What's the difference between.. and ...?