What is the difference between "lots" and "a lot of"?

As topic.

They are basically the same, only "lots of" is slightly more colloquial. Of course in a formal context you would use "many" or "much" instead of "a lot of", but that's the general adjective which can be used for both countable and uncountable nouns in more relaxed speech. "Lots of" is used in casual conversations, or at the end of informal letters, as in "lots of love".

Lots of = plural
a lot = singular

I usually use "a lot of" more often than "many" or "much", but I almost never use "lots of".
I see that most native English speakers spell "a lot" as one word ("alot"). Don't know why.