A cassock is a plain, lightweight, ankle-length garment with long sleeves, but no hood. The cassock is a clerical, not a vestment. It serves as an undergarment for vestments, namely the surplice (a type of alb) and the stole.
Cassocks are worn by both clergy and lay worship leaders, with or without a surplice. Only ordained clergy wear a stole over the surplice.