The slide, or the bottleneck as some call them, is just a tube of glass or metal you put on your finger. Well, actually it could be almost anything but that's the common stuff. Some samples here: http://www.diamondbottlenecks.com/
Normally, to get a certain tone you push the string with a finger to bend it slightly over one of the frets. Now with a slide you have a "moving fret" that you use instead of the regular frets. So you don't use standard already set fret positions, it get's more like a violin in the sense that the string is fretted wherever you touch it with the slide. And chords are played more like barre chords, obviously.
Did that make any sense? Was it even correct? I'm no pro at this either