What differentiates the act of simply grouping people from the act of stereotyping?

Answer 1

Check these articles.

http://www.colorado.edu/conflict/peace/problem/stereoty.htm

Answer 2

Stereotyping assumes that all members of a group have the same characteristics, e.g., "Men, they're all alike!"

http://www.members.cox.net/dagershaw/lol/Prejudice.html

I hope this helps a little more. Thanks for asking.