The walk is the slowest of a horse's paces. It is a four-time movement in which each footfall is distinct. The sequence is left rear, left front, right rear, right front.
Collected and Extended Paces
The horse doesn't have just one speed at each of the paces. He has a 'working' pace which is a natural, steady movement at any of the paces.
If you liken the horse's body to a spring, in a collected pace it is like compressing the spring - the hind legs come well underneath and the neck comes back. The steps are short and springy with lightness.
In an extended pace it is as though the spring has been stretched - more ground is covered with each pace and the body of the horse is elongated.
Next - the trot.