Scratching

Cats are naturally driven to periodically hook their front claws into suitable surfaces and pull backwards, in order to clean the claws and remove the worn outer sheath as well as exercise and stretch their muscles. This scratching behavior seems enjoyable to the cat, and even declawed cats will go through elaborate scratching routines with every evidence of great satisfaction, despite the total lack of results. Indoor cats benefit from being provided with a scratching post so that they are less likely to use carpet or furniture which they can easily ruin.[49] Commercial scratching posts typically are covered in carpeting or upholstery, but some authorities advise against this practice, as not making it clear to the cat which surfaces are permissible and which are not; they suggest using a plain wooden surface, or reversing the carpeting on the posts so that the rougher texture of the carpet backing is a more attractive alternative to the cat than the floor covering. Scratching posts made of sisal rope or corrugated cardboard are also commonly found. Some indoor cats, however, especially those that were taken as kittens from feral colonies, may not understand the concept of a scratching post, and as a result will ignore it.

Source : http://en.wikipedia.org/wiki/Cat#Habitat