Check out Who and That by rogersgeorge. Here is an excerpt:
Here’s the rule: “Who” refers to people, “that” refers to everything else. So you say,
The person who sits next to me stinks.
The person that sits next to me stinks.
Okay, maybe you shouldn’t say either sentence, but you get the idea.
I ran into an exception that works. Maybe we could call…
The full article is available here.