Basically a non-descript nylon bag with minimal accoutrements. I curse its lack of features (extra pockets, etc) on every trip, then it goes on the shelf until the next trip. It was on sale at the local sporting goods store when I was in high school. I recently bought a cheap waterproof cover for it, because I noticed that most of the advertised "waterproof" packs come with a cover.
I have a very lightweight, tiny day pack that I either roll up and stuff in the main pack, or carry on as my "personal item." That way, I can leave the big pack in my hotel room.
