Wolves howl for several reasons.

Howling helps pack members keep in touch, allowing them to effectively communicate in thickly forested areas or over great distances.
Furthermore, howling helps to summon pack members to a specific location.
Howling can also serve as a declaration of territory, as portrayed by a dominant wolf's tendency to respond to a human imitation of a "rival" individual in an area that the wolf considers its own.
This behavior is also stimulated when a pack has something to protect, such as a fresh kill. As a rule of thumb, large packs will more readily draw attention to themselves than will smaller packs. Adjacent packs may respond to each others' howls, which can mean trouble for the smaller of the two. Thus, wolves tend to howl with great care.

Shaun and the Wolves take part in a Howling. Learning from the pack is what it is all about and communication is the key to understanding more about the wolves intricate lives.

Wolves will also howl for communal reasons. Some scientists speculate that such group sessions strengthen the wolves' social bonds and camaraderie— similar to community singing among humans. During such choral sessions, wolves will howl at different tones and varying pitches, which tends to prevent a listener from accurately estimating the number of wolves involved. This concealment of numbers makes a listening rival pack wary of what action to take. For example, confrontation could mean bad news if the rival pack gravely underestimates the howling pack's numbers.

Observations of wolf packs suggest that howling occurs most often during the twilight hours, preceding the adults' departure to the hunt and following their return. Studies also show that wolves howl more frequently during the breeding season and subsequent rearing process. The pups themselves begin howling soon after emerging from their dens and can be provoked into howling sessions relatively easily over the following two months. Such indiscriminate howling usually has a communicative intent, and has no adverse consequences so early in a wolf's life. Howling becomes less indiscriminate as wolves learn to distinguish howling pack members from rival wolves.

Growling, used in tandem with bared teeth, is the most visual and effective warning wolves use. Wolf growls have a distinct, deep, bass-like quality, and are used much of the time as a threat, though they are not always necessarily used for defense. Wolves will also growl at other wolves while being aggressively dominant.

Wolves also bark, which they do when nervous or to warn other wolves of danger. Wolves bark very discreetly, and will not generally bark loudly or repeatedly as dogs do; rather, they use a low-key, breathy "whuf" sound to get attention immediately from other wolves. Wolves will also "bark-howl" by adding a brief howl to the end of a bark. Wolves bark-howl for the same reasons they normally bark. Generally, pups bark and bark-howl much more frequently than adults, using such vocalizations as cries for attention, care, or food.

Wolves can also whimper, which they usually do only while submitting to other wolves.
Wolf pups will whimper when they need a reassurance of security from their parents or other wolves.

All data on this page was taken from http://en.wikipedia.org/wiki/Wolf