When you point at a kitten or you point at a picture of a kitten, you say, "This is a kitten".
When someone else points to a kitten and asks you, "What is this?" you say, "It is a kitten".
This is a kitten. ~ The thing that I am pointing at is a kitten. What is this? ~ What is the thing that I am pointing at? It is a kitten. ~ The thing that you just mentioned (the thing