In this case, they mean the same thing - "many" and "a lot of" are synonyms. However, "many" only works with countable nouns e.g. "I' have many food" is wrong. "I have a lot of food" is correct. Hope this helps!

With the word 'many' in your statement, you would expect to be able to number the people. For example - 'Many people were standing near the house. There were at least 1,000 of them.' With the word 'lots' in a statement, you would follow up with an additional 'lots' statement. For example - 'Lots of people were standing near the house and lots of reporters were there too'. You could write 'Lots of people were standing near the house and many of them were reporters'. This would act as a 'classifier' of the unknown number of people, with a large number being reporters.