[Obbiesan]

well kinda but they are called greifers. most servers will have the ability to lock chests via mods for the game. Greifers will destroy your house / structure, But again most servers have protecting against this. As for in game monsters nothing will steal you stuff but Creepers will destroy it =/= i hate them so much XD.

A creeper is basically a green rectangular monster that will walk towards you and explode when it gets within a certain distance of you. you can kill them with swords and some skill XD