Do you mean 'a lot' and 'lots'? Note that 'a lot' is two words. There are a few differences: 1. 'Lots' is slightly more informal than 'a lot'. 2. 'A lot' can be an adverb e.g. "He reads a lot". We don't normally use 'lots' in this way. 3. 'Lots' can sometimes mean 'plenty of' - in the sense of 'no shortage of'. For example: "We'd better hurry. I think we're going to miss the bus" "No, it's fine. We've got lots of time - nearly 20 minutes. Don't worry" We wouldn't us 'a lot of' with that meaning.

I think they are usually used in the same way ("a lot" is 2 words by the way): I ate a lot of food today. vs I ate lots of food today. Lots is usually not written, it's less formal. Also, if you want to use the word at the end of a sentence, just use a lot, not lots: I go to the gym a lot. (sounds much better than) I go to the gym lots.