Meredith Lavine, Artfulresumes.com:
An informational interview is an information gathering meeting to make a new contact and gain valuable information about an organization and their operations. Conducting an informational interview solidifies that industry contact and builds a rapport.
There is no job opening, so the pressure is off and you are given a chance to make a formal introduction and make your capabilities known. An informational interview is a great opportunity for you to be the one in control asking the questions and it is a strong confidence builder. It is best to make an outline of questions you have beforehand to set an agenda and to be sure you can gather the information you need.
Treat an informational interview in the same professional way you would treat a standard job interview. Always follow it up with a thank you note within 24 hours.