Color
The eight colors of Aussies are blue merle (black and gray with white patchwork), red merle (red and beige with white patchwork), black (which may or may not have white legs, a white chest, or a white collar), and red (which may or may not have white legs, a white chest, or a white collar); each of these colors may also have copper points on the eyebrows, cheeks, and/or legs to create four additional combinations. Thus, dogs with copper and white along with the primary color are called tricolor, dogs with white or copper along with the primary color are called bicolor, and dogs with no white or copper are referred to as self-colored. White should not appear on the body of the dog from topmost point of the shoulder blade to the tail except in the merles.

Eyes
There is also great variety in the Aussie's eye color. An early nickname for the breed was "ghost-eye dog". Aussie eyes may be green, hazel, amber, brown, or blue; they may have two different colored eyes, or even have bicolored or "split eyes" (for example, a half-brown, half-blue eye), which appear to be linked to the merle coloration. Merled eyes occur as well, where one color is mixed in and swirled with another. Any combination of eye color is acceptable in the breed standard, so long as the eyes are healthy. In general, however, black Aussies tend to have brown eyes, while red Aussies tend to have amber eyes. Though these Aussies may also carry the blue eyed gene.

Tail
A hallmark of the breed is a short bobbed or docked tail in countries where docking is permitted. Some Aussies are born with naturally short bobbed tails, others with full long tails, and others with natural partial bobs, where the tail is midlength and appears stubby. Breeders have historically docked the tails when the puppies are born. Even without a tail, the wagging movement of the hind end still occurs. Some Australian Shepherd breeders opt to keep the tail on the dog for the natural look, which can still be shown in the breed ring.

Temperament
The Australian Shepherd is unique with regard to its temperament. There are two distinct types of personality to look for depending on the lines, as well as many shades within these two types.

Generally the breed is an energetic dog that requires exercise and enjoys working, whether it is learning and practicing tricks, competing in dog agility, or any other physically and mentally involving activity. Other Aussies would rather be with their humans and enjoying being couch potatoes. It is usually a sweet and affectionate dog who is faithful to its owners and should be good with children. Most Australian Shepherds make wonderful family dogs.

Dogs with strong working instinct may show more reserved, guarding behaviors along with a tendency to chase or nip at running children or strangers if not properly trained. Its protective instinct and behaviors can be frightening to children, strangers, and small animals. Those bred for a more family-oriented temperament are more friendly and affectionate with strangers and generally more reliable around children. Because the breed was developed to serve on the ranch, a job which includes being protective of its property, it sometimes can be annoying with its inclination to bark warnings about neighborhood activity, but it is not generally an obsessively barking dog.

The Aussie is intelligent, learns quickly, and loves to play. This means that a bored, neglected, unexercised Aussie will invent its own games, activities, and jobs, which to a busy owner might appear to be hyperactivity: for example, an Aussie may go from being at rest to running at top speed for several 'laps' around the house before returning to rest, all apparently for no purpose. Without something to amuse them, Aussies often turn destructive of yard or property. Aussies also do best with plenty of human companionship: they are often called "velcro" for their strong desire to always be near their owners and for their tendency to form intense, devoted bonds with select people.

The Australian Shepherd has a reputation as a highly intelligent and versatile stock dog with a range of working styles. While improperly trained or frustrated Aussies may exhibit excessive running and barking, a good working Aussie is quick, thoughtful, and easy with its stock. The ability for the breed to adapt to the situation and think for itself makes it an excellent all-around worker. 
From Wikipedia under the GNU Free Documentation License