Agouti cats can be blotched (or classic) tabby, mackerel tabby, spotted, or ticked (Abyssinian).
A mackerel tabby is, like the name suggests, a cat with vertical relatively small stripes on the body. A classic or blotched tabby has broader bands and the pattern is more marbled. Spotted means that the cat has dots and a ticked cat has a dark stripe along it's back continuing out on the tail tip while the rest of the cat is pure agouti without any markings. Abyssinians and Somali are example o breeds that have ticking, and this pattern is often called Abyssinian.
The three patterns blotched tabby, mackerel and ticked, are the result of three different alleles on the same locus. The gene for ticking is dominant over the other two, and the gene for mackerel is dominant over the gene for classic tabby.
Denominations: Ta = Aby pattern, ticked tabby T = mackerel tabby t (alt. tb ) = classic/blotched tabby
One doesn't know the exact genetics behind a cat becoming spotted. There might be one or more genes breaking up the bands on a tabby or the stripes of a mackerel into spots. It is generally considered that the spotted cats that have tabby as a foundation will have the sharpest dots. It is not unusual for cats to have both stripes and dots, so it might be hard to decide whether a cat is mackerel or spotted.
News! A new theory about the inheritence of the different tabby patterns has been suggested. Read about it here, in Dr. Lorimer's website!