A slide can be made from metal or glass and is worn on the middle, ring, or pinky finger of the fretting hand.
Playing with a slide allows you to seamlessly move between notes without needing to press down on the frets and gives the guitar an expressive, vocal quality.
Notable slide guitarists: Elmore James, Duane Allman, Ry Cooder, Bonnie Raitt, Johnny Winter, Derek Trucks, Warren Haynes, Ariel Posen, Joey Landreth
For more, check out Ariel Posen's Slide Guitar Pickup Master Class.