Birds are typically categorized by their shape and physical characteristics. For example, shorebirds have long legs and necks and upright bodies while gulls have short necks and are about as long as they are tall. Dividing birds into groups is an important skill, especially if you plan to use field guides for identification. Field guides group birds, so if you can decide which group a bird is in, you will more easily be able to use field marks to determine which species you are looking at. You can find silhouettes like those seen above at the front of almost any field guide.
(Courtesy of The Cornell Lab of Ornithology)