A glass ball the size of a large marble. In its default state (when nobody is touching it), a Remembrall is full of white smoke. When picked up and held in a person's hand, a Remembrall will glow scarlet if there's something the person has forgotten to do.
The name "Remembrall" = "remember all", and also suggests the object's ball shape.
Neville's Gran sent him a Remembrall by owl post during his first term at Hogwarts; Draco's theft of this as a practical joke led directly to Harry being selected as Seeker for the Gryffindor House team (PS9).
See RESURRECTION STONE.