These three expressions are used in informal English. They can mean either a great quantity of or a large number of and can be rather confusing at times. Here are the general rules for their use.
A Lot Of / Lots Of These two expressions both mean a great deal of or several. They are used before a count or non-count noun. These two expressions tend to be used in informal English.
Examples: We need a lot of people for this game. She likes lots of jam on her toast.
A Lot Use a lot at the end of a sentence as an adverb. A lot is NOT followed by a noun. The meaning is the same as a great deal.
Examples: I enjoy swimming a lot. Mary seems to travel a lot.