Espalier is basically controlling the growth of a plant or tree so that it grows into a shape of your choosing. That is really simplifying it (so feel free to pipe in, all of you espalier experts), but the basics are covered in my father's Rose of Sharon flower wall (seen in the pictures).
He started with regular, young Rose of Sharon. These usually grow like a small-ish, round tree (here are a few pictures of what they generally look like). He planted them at the base of our porch and began tying branches of the Rose of Sharon to the posts of the porch until the branches were flat against the posts. As the plant grew, he again tied the new growth to higher porch posts, so that the Rose of Sharon was now growing tall and flat, like the side of the porch, instead of round & bushy. Here it is from the back. You can see how the branches are tied flat against the porch posts:
Now, a few years later, the Rose of Sharon acts as outdoor wallpaper on the porch. It's a beautiful backdrop to our family dinners and is growing taller every year. My dad trims it back occasionally and ties branches that are growing out of place back to the porch.
Here's a shot from the third floor of our house (is it weird that I still call it "our house" even though I moved out 10 years ago?):
|I spy my cement planter...