Ever wondered why I build those super-expensive 4-legged robots with 12 servos, three servo per leg, when all you need to have a "walking" quadruped robot is two servos and a piece of wire bent just the right way, and three servos for a hexapod? My answer is simple: those robots are not really walking. They are crawling. What's the difference?
When you are walking, you make steps -- your feet move, relative to the ground, only when they are in the air. Once you put a foot on the ground, it stays pretty much in the same spot. You don't drag your feet.
When you are crawling, most of your body stays in contact with the ground, and you use your arms and legs to push it forward. It's similar to rowing. You never completely raise from the ground, some parts are always touching it, you just drag them.
But what is the practical difference? First of all, crawling usually requires much more energy. How much exactly depends on the kind of surface you are crawling on -- which is another down side. You can walk as well on a puffy carpet as on a smooth floor, but crawling on the carpet is much harder. Then, crawling doesn't require much balancing. Since most of your contact surface stays the same, there are no complex movements you need to do to shift your center of gravity to keep from falling down. For all you care, you are already down. And finally, most importantly to me, crawling is not nearly as interesting.
What about those single-motor mechanisms that use intricate systems of levers or gears to move their many legs in a walking fashion? In theory they lift their feet for movement, and they don't drag them on the floor much, so that counts as walking. However, in practice, this walking doesn't give them any advantage over normal wheeled locomotion. In fact, if you look carefully, they simply move on wheels, just the wheels touch the floor through the system of levers, instead of directly. This of course looks cool and all that, but it doesn't let you move more efficiently over uneven terrain, climb stairs, etc. -- so most advantages of walking are gone. They still make nice toys, though.