Learn French: C'est un chat. Il est très petit. - It is a cat. It is very small.

This is the main difference between identifying and describing! Use c'est when you are identifying a thing with a noun phrase, like un chat (a cat). Use il est (or elle est) when you are just describing the thing with an adjective, like petit (small).

Why does the second sentence use Il for an animal? Doesn't il mean 'he'?

French doesn't have a neutral 'it' pronoun like English does. Every noun in French is either masculine or feminine. Because un chat is a masculine noun, you must use the masculine pronoun il to refer back to it, even if it's an animal or a lifeless object.

If we were talking about a female cat, how would these sentences change?

First, the noun phrase would change to feminine: C'est une chatte. Then, the pronoun and the adjective in the second sentence would also need to match that feminine gender, becoming Elle est très petite (notice the extra 'e' added to the end of petite to make it feminine).

Are the 't's at the end of the words in this sentence pronounced?