Woodpeckers are robustly built birds. They have a unique anatomy that enables them to exploit habitat and food niches that are unavailable to most other birds, and indeed many mammals. They have evolved many anatomical and structural features that allow them to cling to and climb up and down wood surfaces (which are often smooth) and hack into timber. These features include muscular bodies, tough skin, enlarged and adapted skulls, relatively large ribs, broad tendons, ridged chisel-shaped bills, strong feet, sharp claws, particularly strong tail feathers, and many others.
Many of these features can be seen well on this photo of Great Spotted Woodpecker here, taken in Sussex, England by David Plummer: http://www.davidplummerimages.co.uk/
Note the broad chisel-shaped bill with a sharp tip, The robust legs and feet with long sharp claws and the overall stout, strong body.