"Crow Duck" is a fairly accurate description, as the cormorant appears to be a cross between a crow and a duck.  A dark bird, it has a long black neck with a brightly colored beak.  Both the male and the female look similar, the major difference being during mating season, when the "crests" on the head of the male become apparent.  Cormorants are often observed extending their long black wings (app. 50 inches) to dry.
Source: Canadian Wildlife Service Cormorant Fact Sheet

Natural History
The mating dance of the male cormorant is an intricate one, including seduction of the female with nesting materials.  Double-crested Cormorants maintain a clutch of about 3-4 eggs.  These eggs hatch after about 28 days of incubation and then are fed and attended to by both parents.  These are migratory birds.  They winter along the Mississippi Delta, where they are also considered to be a nuisance species, primarily to aquaculturists.  Aquacultural farms are easy targets for a food source.

 Photo Source: Eleanor Briccetti

There are three impacts primarily associated with the Double-crested Cormorant.  The primary and most documented impact regards the vegetation associated with colonies of breeding cormorants.  Cormorants nest high in the trees.  The acidic nature of the cormorant’s feces kills the surrounding vegetation and eventually leads to the death of the nesting tree. However, this is of little consequence to most managers because there is no shortage of vegetation in the Great Lakes Region.  The form of vegetation that managers are concerned about is the Carolinian vegetation.  Carolinian vegetation is the rarest form of flora in the Great Lakes region (Weseloh et al., 2002, p. 135). 
The second impact may be on other colonial water birds.  Species such as black crown night herons (Nycticorax nycticorax) nest in the understory of trees.  Of course, this tends to be the first part destroyed by the cormorant droppings.  Cormorants also have a tendency to “hi-jack” the nest of these other species between nesting seasons (Weseloh et al., 2002, p. 135). 
The third impact is one raised by an important element of the Great Lakes economy.  The sport fishermen surrounding this region claim that Double-crested Cormorants have a great impact on fish populations.  Anglers insist that the cormorants tend to favor fish that are commonly regarded as prize catches.  These species include walleye, yellow perch and smallmouth bass (Weseloh et al., 2002, p. 136).  However, the vast majority of scientific studies have shown that the statistics that the anglers are basing their information on are somewhat skewed.  Researchers claim that the birds' impact, is dependent on the size of the body of water and the available populations of fish.  Studies in the region have proposed that cormorants are “opportunistic foragers."  This indicates that the birds eat whatever they can find, and they do not search out prey with respect to any criteria except whether or not they can fit it in their beak! (Stepanian, 2002, p. 119) 

Executive summary to the US Fish and Wildlife Service on the Status of the Double-crested Cormorant in North America. Link
Includes an interesting list of reasons why population numbers have grown so rapidly.  Brief explanations of impacts of the bird and management options. 

US Fish and Wildlife Service Links:
The Great Lakes Basin Ecosystem Team  Link
Gives great leads to other sites regarding cormorant management

The Official USFWS Double-crested Cormorant site.   Link
Includes the draft environmental impact statement, general information, research being done, and management impact. 
 

Great Lakes Information Network Link
This is a great page to lead educators through the process of teaching invasive species.  It also contains extensive information if the educator is unfamiliar with the subject. 

Weseloh, D.V. and Collier, B.  The Rise of the Double-crested Cormorant on the Great Lakes: Winning the War Against Contaminants, a Great Lakes Fact Sheet. The Canadian Wildlife Service. http://www.on.ec.gc.ca/wildlife/
factsheets/fs_cormorants-e.html.  Updated 5/31/00.

Stepanian, Martin A. (2002).  Interspecific Interactions, Habitat Use and Management of the Double-crested Cormorants (Phalacrocorax auritus) in the Laurentian Great Lakes: An Introduction.  Journal of Great Lakes Research. 28(2): 119-124.

Weseloh DV, Pekarik C., Havelka C.T., Barret G., and Reid J. (2002).  Population Trends and Colony Locations of Double-crested Cormorants in the Canadian Great Lakes and Immediately Adjacent Areas, 1990-2000: A Manager's Guide.  Journal of Great Lakes Research. 28(2): 125-144.