An espalier is a tree or shrub pruned to grow in a flat, two-dimensional plane. Typically they are used against walls or fences. Historically, espaliers were often fruit trees, especially apples, because their shape allows the remaining leaves to photosynthesize efficiently, ripening fruit quickly. They are ideal for small gardens and courtyards.
I think this is a particularly San Diegan espalier: an Opuntia (prickly pear cactus) I spotted in South Park. I'm not sure it would have occurred to me, but I like it! After all, they may be a lot of work but prickly pear is edible, either as the fruits, or as nopales.
Here is a more standard espalier of an apple tree:
Other good options for an espalier: figs, lemons, limes, satsumas, plums, and peaches. If you have seen another interesting espalier, let us know!