Tabbies come in two basic colorations - red, brown or ginger tabbies (with a rich orange/gold/red coat), and silver or grey tabbies (coat colouring including silver/grey/black). Black tabbies (as in the picture above) are often wrongly called "brown tabbies" (especially in the US), but they are genetically black and have nothing to do with the "brown" (chocolate) gene, so the name can be misleading.
The tabby stripes can either take the form of a distinct mackerel or fishbone striping, or they may be more loosely splotched. Alternatively, they may have a spotted coat, which is actually stripes broken up into distinct blotches, or they may be ticked. This is where the hairs have distinct bands of colour on them, breaking the tabby patterning up into a salt-and-pepper appearance.
Many tabbies have a distinct 'M' marking on their forehead--the mark of the true tabby. There are several legends about where this came from. Also, all tabby cats have a brick red or rose-colored nose and light-colored areas around the eyes.
Until the 16th century, the spotted tabby with its wild ancestral markings of spots and stripes was the dominant tabby type. During the early 1500s a variant of the spotted tabby appeared, now known as the mackerel tabby with its striped coat. These markings then became the dominant tabby pattern. The spotted tabby, while not uncommon, is now usually found only in purebred animals.