They mean the same thing- often they are interchangeable, however depending on the sentence sometimes it is more appropriate to use one over the other.
eg. There are lots of words in this sentence.
There are a lot of words in this sentence.
Both are correct,
As a general rule if you are emphasizing the intensity of something you would say there is a lot of. If you are emphasizing the quantity you would say there are lots of.
eg. There is a lot of pain in my right shoulder.
This is better than 'There is lots of pain in my right shoulder' as pain is experienced as a singular entity and 'a lot' refers to the intensity of the pain.
eg. There is a lot of negativity in your voice.
Again here it is saying that the negativity in your voice is intense, rather than there are lots of instances of negativity in your voice.
eg. There are lots of different colours in the painting.
Here lots is used to draw your attention to the vast quantity
eg. There are lots of spelling errors in your essay
Again the focus is the number of spelling errors.
However, this is a nuance, and everyone would understand you whichever you chose to use. I really wouldn't stress over which is better.