A wooden crate has a self-supporting structure, with or without sheathing. For a wooden container to be a crate, all six of its sides must be put in place to result in the rated strength of the container.
Crates are distinct from wooden boxes. The strength of a wooden box is rated based on the weight it can carry before the top (top, ends and sides) is installed, whereas the strength of a crate is rated with the top in place.
In general conversation, the term crate is sometimes used to denote a wooden box.