To be perfectly honest, I'm not entirely sure where Joshua trees fall on the range of tree to cactus. However, they have tree in their name, so it's just going to have to count today!
Joshua trees can only be found in a pretty small swatch of the southwest, concentrated in the Mojave desert. I think that Joshua trees kind of look like they belong in a Dr. Seuss book with their spiky poofs of leaves. The place to really experience them though is Joshua Tree National Park. While you're at the park you can wander through forests of Joshua trees, or pick your way up to the top of one of the ginormous boulder piles. Look closely and you may see some smaller cacti blooming and use sun protection while you're there... I learned that one from experience!