Espalier refers to the practice of tying the branches of trees to some sort of flat support (a wall or trellis, for example), creating formal, geometric patterns. The use of espaliered trees isn't reserved for formal gardens, but are a great way to get a high fruit yield in a very limited space. And it's a very graceful way to liven up a boring or unsightly wall or fence without fear of the tree getting out of control the way a vine would.
One huge advantage of espalier, is that it in cooler climates, it allows warmer weather species of trees to grow well because they absorb as much sunlight as untrained trees. This method can be very high maintenance, so you might want to limit how many you plant. But if you're up for the challenge and work, it could make for a very striking and bountiful garden.
You can either completely DIY the training of the branches via tutorials from sites like Vintage Garden Gal or Gardening Know How, or you can purchase a tree from a local grower like Flowering Shrub Farm, where they've started the training for you.
Do you have any experience with espaliered fruit trees? Love them? Hate them? Tell us about it in the comments.