BruceMol has given you a great resource to find what the "typical" definition of a screener is. It will get you most of the way there. Thanks man!
For what it's worth, I have had to give many PBS stations and film festivals "screeners" of some of the films that we have made at the video lab where I work. For us, this means giving the station managers or event organizers a DVD that has a watermark on it at full SD (720×480) quality, without a menu system (the video is set to the "First Play"), and of course, a press kit just as BruceMol suggested (a press kit is crucial to making sure the right people know how to contact you).
A screener is just meant to give station managers and event organizers an idea of what the content of your film is and if the audio and video quality are up to their standards. It can also be used to see if the message fits with a particular station or event. Basically, it is a no-frills "first look" at a film on a standard DVD so that a station or event can determine if they want to air it. As such, make sure the film has been polished off, but just give them a simple DVD video with a watermark on it (if you're worried about pirating, though I've seen plenty of pirated DVD screeners on the interwebs!)
Hope that helps!