I had never heard of an "informational interview" until I went through a job search workshop. The idea sounded great, though. The facilitator made sure we understood we were not to go in and ask if there were any jobs for us, but that we were to get information about the company, division, industry, key people, etc.
In short, it was an interview where we gathered information that could help us understand the market and needs better. Sometimes the premise of getting the informational interview would be to figure out if we would really be interested in a major career change (to a different industry, for example). It really was about collecting information, not asking for a job.