A digitigrade is an animal that stands or walks on its digits, or toes. Digitigrades include walking birds (what many assume to be bird knees are actually ankles), cats, dogs, and many other mammals, but not plantigrades or unguligrades. Digitigrades generally move more quickly and quietly than other animals.
There are anatomical differences between the limbs of plantigrades, like humans, and both unguligrade and digitigrade limbs. Digitigrade and unguligrade animals have relatively long carpals and tarsals, and the bones which would correspond to the human ankle are thus set much higher in the limb than in a human. In a digitigrade animal, this effectively lengthens the foot, so much so that what are often thought of as a digitigrade animal's "hands" and "feet" correspond only to what would be the bones of the human finger or toe.
Humans usually walk with the soles of their feet on the ground, in plantigrade locomotion. In contrast, digitigrade animals walk on their distal and intermediate phalanges. Digitigrade locomotion is responsible for the distinctive hooked shape of dog legs.
Unguligrade animals, such as horses and cattle, walk only on the distal-most tips of their digits, while in digitigrade animals, more than one segment of the digit makes contact with the ground, either directly (as in birds) or via paw-pads (as in dogs).