Open is a catch-all; basically any interp you want to do, including your own writing, and multiple interps all worked together around a common theme (called a thematic. There was one year where we had a thematic category separate from Open, which was... interesting) as opposed to Humorous Interp (should be funny) and Dramatic Interp (should be dramatic, but in our area, it generally ends up just being morbid....)

Expository is a platform speech where you get to use props. In my case, that means a bunch of Kinkos-printed powerpoint slides with vikings and Shakespeare.