What's the difference between an outline and a treatment?

An outline is generally a document created by a writer for the writer to perfect his/her story before penning a full script. It usually contains less detail then a treatment which is more of a presentation document used to sell a film idea to a studio or production company.

However, an outline can also be used for presentation purposes and is also extremely helpful when rewriting a project and creating a "beat sheet" or scene by scene breakdown of a story or script.