The first slides were made from the necks of glass bottles so this technique is also known as "bottleneck guitar". Short slides are available but most people use full length slides as they allow you to play bigger chords.

Intonation is about playing in tune. The slide should be positioned directly above the frets. The slide merely rests on the strings as they don't need to be pushed down onto the fingerboard. Alternate tunings are often used as they minimise the amount of fingering required to make chord shapes.