Here are a few links to get you started:
Permies thread on converting orchard to food forest
Another page on food forests
Very basically- an orchard tends to be a monoculture tree crop, or maybe a mixed crop. Think of an apple orchard- evenly spaced trees, pruned to a very specific height and shape, with very little else growing. Some orchards will be mixed- apples and pears for instance. Or fruits and nuts growing together.
A food forest is a multi-layered planting that includes a variety of different food trees (fruits, nuts) as well as shade, medicinal, N-fixers, and mulch trees. Also, vines growing up the trees, bushes on the ground, root crops, herbs, etc. All growing on different levels.
An orchard is easily legible- you look out and see that there is one thing growing out there and it's all ripe at the same time and it's lined up nice and neat.
A food forest is illegible if you don't know what you're looking for. Multi-layered, multi-species, and very complex.