There is - This is; There are - These are
There is a cat. This is a cat.
There are cats. These are cats.
Difference?

Question

Answer

This + These = near to you
That + Those = far from you

Here = near to you
There = far from you

"Do you want this shirt here or that shirt there?"

Answer

"There is/there are" can be used when you mean that something exists. This is very common in English. By the way, you wouldn't say "cats exist" instead... that simply sounds very strange.

Those phrases can also mean "not here, but a little further away".

Use "this" and "these" (as well as "that and "those") when you are indicating the cat(s). Imagine yourself pointing at it/them.

Answer

There is and there are refer to something that is further away, or hypothetical. Whereas this is and these are refer to something physical which is close by.