College Football rules question

[Vanilla Bear]

I was watching a bit of a College Football game earlier and I noticed something.

A WR caught a ball and hit the ground with his knee and elbow and the play was over right away without the receiver being touched by the defender. Isn't the Receiver allowed to touch the ground with his knee or elbow in College Football? He doesn't need to be touched by a defender to be down by contact like in the NFL?

Are there any other major differences in College and NFL rules?

Thanks for any input.