"a lot of / lots of" - A lot of and lots of have the same meaning, they both mean a large amount or number of people or things. They are both used before countable nouns and uncountable nouns. As they are similar it can get quite confusing. A lot of people went to the game. Lots of people went to the game. A lot of snow falls in winter. Lots of snow falls in winter.

There is no difference. They are the same word.

I think informally, they have the same meaning. However, the phrase "a lot" refers to an indeterminate unit of measure (which may in certain contexts actually be determinate, e.g. land measurements). So, technically, "lots of" should be more than "a lot of", since the latter refers to a single unit of measure, and the former refers to multiple units of the same measure (if used in the same context).

ElTempo is right. There is no difference between (a lot of ) and (lots of).