The standard for each breed is specifically drawn up to set out guidelines covering the desired externally observable qualities for that particular breed – its physical conformation, character and temperament.  You can read more about it here.

Why are they important?

The breed standard provides a working outline of the desirable traits and hallmarks of the breed, pushing healthy sound dogs. Movement, temperament and soundness, including breathing are highlighted in the standard. The standard is the blueprint which defines each breed individually. When judging, one is selecting the best representatives of the breed, ie the best specimens to breed on with. It is not a beauty contest.