Walk is a symetrical, lateral gait with four beats.
The move of a hindlimb is followed by the move of a forelimb on the same side, which gives the following beats:
right hindlimb -> right forelimb -> left hindlimb -> left forelimb
In the case of a dog walking very slowly, the is one foot in the air and three feet on the ground at any point in time.
When the speed increases, the time shift between the limbs slightly changes, and their time on the ground decreases, so there can be two feet in the air at some point in time. hence, for a dog walking at normal speed, there are alternately two or three feet on the ground.
If speed increases more, the phases with three feet on the fround disappear, giving a gait that one can be mixed up with the horse amble. But for the dog this gait is generally transitory between walk and trot.