Wolves howl for a number of reasons.
They will howl when they are away from the pack to stay in contact with the rest of the pack.
Wolves arevery territorial and for a wolf to be caught away from the rest of the pack could be fatal.
Despite popular beliefs, wolves do not howl atthe moon even though it may appear that they do.
WOlves howl on nights when the moon is not visible just like they do on nights that the moon is visible.
Here is a link that has a lot of information on why wolves howl.
Wolves not only howl, they have a very large vocabulary and make a large range of sounds.