Dogs have a similar structure of bones as we do but they stand on their tiptoes, which is why they have their cushioned foot pads to protect the bones. Their front knees are the equivalent to our wrists and their hocks in the hind leg are the equivalent to our ankles.
Dogs are designed as us to have many small bones at the end of their limb (hand or foot) so that should they fall from a height, hopefully it will be a toe that breaks, if anything. Next would be the wrist or ankle, then the lower leg bones. A more severe fracture would be an upper leg fracture (the femur bone or humerus). Bone is like willow and is pliable, a dog that is young and fit is less likely to fracture a bone than an older dog or a younger dog with a congenital problem such as IOHC or OCD.
They walk on four toes and the dew claws are the equivalent to our thumbs. Some breeds also have a dew claw at the back. Most of the weight is taken through the central two digits. dogs have a large, heart shaped central pad under the end of their metacarpal bones (metatarsals in the hind leg), which on us, would be the area where the fingers meet the palm. This surface of the paw bears the weight of the dog for most of the time. There are an additional 4 oval shaped pads under the distal joints of the toes (on us, this would be the first joint from the fingernail, on the opposite side to the nail). The pads are thickened epidermis (like skin) and have concentrated sweat glands that are likely to play a part in territorial marking.
Although the dewclaws appear to have no use, they are useful to the dog and are used as thumbs. The dog will use it to hold food or toys in place while they are eating or playing with them. They also would use them to gain purchase to drag themselves out of danger as mini pick axes. You have probably noticed the pad further up the leg that also looks as if it has no purpose - this is called the 'stopper' pad or carpal pad.
When the dog is running, the dew claws can come into contact with the ground and can dig into the ground to stabilise the leg. This is of particular importance if the dog turns as it is running as it reduces torque (the twisting force that leads to rotation) to the knee and elbow. It is also thought that the stopper pad also come into contact briefly with the ground when the dog is in canter (running really fast). Should the dog stop suddenly or turn quickly in canter, the dew claw and stop pad will work together to reduce the chance on injury to the knee.
If you look at the picture below, you will see that their nails literally sit around the end of their distal toes. The digital pad at the bottom of the foot is one of the four oval pads.
As you can imagine, the pads that the dog weight bears through can be extremely painful for the dog if they are injured and will lead to lameness. Grass seeds can pierce the pad and if not found, can migrate up the arm to cause more discomfort. Interdigital cysts can form inbetween the pads to cause discomfort and cuts can be very uncomfortable for your dog. The picture below show an interdigital cyst (interdigital furuncle). These are large bumps, or nodules, between a dog's toes. Cuts can be very uncomfortable for your dog as are torn nails.