Emily, there is no such thing as a 'bonsai' (not bonZai) tree, only regular trees that have been trained using bonsai techniques. If you're thinking of the usual juniper/evergreen with an "S" curve as being bonsai, you're not alone, but it's just some regular tree that's been worked on. If you saw a willow tree in a bonsai'd shape, then it was a willow bonsai, and if you think it wasn't a bonsai because it was larger than a few inches in a small pot, again that's just common belief, but many famous bonsai are quite large = 3-4' high, and the same across, but they'll always be in a container/bonsai pot, and not planted in the ground... even if bonsai techniques were used to prune them.

I discovered I was looking at a Japanese Maple - beautiful.