A craven man is no Superman or Spiderman, nor is he a firefighter or a soldier. A craven man is the opposite of those guys: he has not an ounce of courage.
In "The Wizard of Oz," the Cowardly Lion could have been called the Cravenly Lion, but that didn't sound quite right. Use craven as you would cowardly. A craven leader is scared to lead, while a craven gymnast stays on the mat and avoids the balance beam. You can also use the word to describe other things, besides humans. Craven policies, for example, are probably weak and do not take bold measures.