They are used to express the existance of something, just to show it.
"There is" is used with singular elements and "There are" with plural elements.
- There is a cat on the roof.
- There are cats on the roof.
They work as any sentence with the verb "to be", so they make direct negatives and subject-verb inversion in the interrogatives.
- There isn't a cat on the roof.
- There aren't cats on the roof.
- Is there a cat on the roof?
- Are there cats on the roof?