In this image, the bucket is a sprite, whereas the barrels are not.
A sprite is a two-dimensional image which is projected on the three-dimensional world of RuneScape Classic.
Players, items, monsters, and non-player characters are represented with sprites. The scenery and landscape are not.
Walls, while appearing to be sprites, are actually textures projected onto thin three-dimensional objects. They provide full viewing angles and do not appear to "snap" around or stay still as the camera is rotated.