what was the function of the Roman cavalry?

[Juul]

What was the function of the Roman cavalry? I'm trying to answer that question, but it doesn't work out very well. I'm searching for information about horses in the roman army. I've found out that the cavalry -compared to the infantry- wasn't very big. So I guess it wasn't important. I found out most cavalry was part of the Auxilia, but I just can't find what the real function of the cavalry was.

I hope someone can help me finding this information, or just tell me a bit more.

(Books aren't an option, because most books are in English and it ain't possible to get them here.)