Q:

What's the term for the part of the URL after the question mark?

http://www.example.com?foo
What's the term for the foo part of the URL?

A:

Its the query, or sometimes the query string.
To pinch a useful diagram from the URI RFC:
     foo://example.com:8042/over/there?name=ferret#nose
     \_/   \______________/\_________/ \_________/ \__/
      |           |            |            |        |
   scheme     authority       path        query   fragment

A:

It's called "query string" as you can see in wikipedia.

A:

The "foo" part of the URL, as you put is, is referred to as the query.
source:
http://tools.ietf.org/html/rfc3986#section-3.4

