Object

Object is not one of the three classifications of monsters in Diablo II. All monsters belong to one of the classifications in Diablo II, and certain spells or items are more effective against different types of monsters.

Objects are enemies, but not really monsters. Objects include things like traps, barricades, sarcophagi, blood pits, torture machines, monster generators, and many more. Most objects act as monster generators, creating new enemies all at once or in a steady stream over time. Such objects can usually be destroyed, ending the flow of new enemies.

• Beasts (category) - Feral, unintelligent enemies. (Called "Animals" in Diablo II.)
• Demons (category) - Mostly Sinister, sentient monsters from Hell.
• Objects (category) - Stationary obstacles, like traps.
• Undead (category) - Varying types of re-animated corpses, mostly from humans.

For a full list of Objects, check out the Objects Category. You can also go to the Diablo 2 Wiki to the find Diablo 2 Objects.

Treasure Chests

The game developers also use the term "objects" to refer to treasure chests, barrels, crates, dead bodies, and other clickables. These are objects that players can interact with by clicking them, potentially turning up gold or items. Every map has hundreds of manually-set locations for these sorts of objects to appear. Objects will not appear in all those locations each game, but in some percentage of them. This lets the chests and other clickables vary between games, but doesn't put them into truly random locations, such as where they might be inaccessible, or block a hallway.

This system was described in the Crafting Sanctuary panel at Blizzcon 2010, where the image below was taken from.

Potential treasure "object" locations on a map.