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

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

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

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