I agree with Anna. These words mean the same thing. Also I agree that "a lot of " is more formal than lots of, or lotsa. Please keep in mind that lotsa can definitely be heard in different forms of spoken English. It is natural but not written. A lot of takes longer to say. If you are giving a speech and you want the listener to really pay attention to what comes after a lot of then separating the words makes it longer and so the listener might be more drawn towards what comes next. I will record my voice and give you an example. But again this doesn't change the meaning.

Hi Jahidul, Lots of is more informal than a lot of. A lot of and lots of can both be used with plural countable nouns and with singular uncountable nouns; they're also interchangeable.

They are the same except "a lot of" is singular (one) and "lots of" is plural (more than one) It would be similar to telling someone, "I hope you have a ton of fun!" Versus "I hope you have tons of fun!" The only difference is one is singular and the other is plural when referring to "ton"/"tons".

they are interchangeable