Tanpoint

Tanpoint, which I will here divide into three basic patterns, is a fairly recessive but common worldwide mutation on the A locus. Genetically, it is known as at and is the 2nd most recessive gene on its locus (only dominant over recessive black). It is however very common and so is found in many breeds. To exhibit tanpoint, the dog will also need to have kk on the K-locus, which allos expression of the A-locus.
Tanpoint is similar to a grizzle, but with the contrasts between the colours made much sharper, to the point that only the lightest and darkest shades remain - a 2-coloured pattern, rather than the gradually banded bi- and tri-coloured hairs of a grizzle. Studying a wolf you will see many different shades throughout the coat, giving gradual borders between light and dark. In a tan-marked dog, there is a crisp and clear border between base colour and markings. The markings are still the same, though you will not see the typical light band known as the harness marking in tantpoints.

A traditional tanpoint looks like your average dobermann, rottweiler or manchester terrier. The base colour can be black, liver, blue or isabella with white-cream to dark red points. 
The tan points normally appear above the eyes, inside the ears, on the lower muzzle and throat, as two symmetrical spots on the forechest, on the insides of the legs, on the paws and under the tail.
In some breeds, due to long fur or perhaps modifying genes, the tanpoints can appear to be less visible than this. They can also be tipped with black, which is often referred to as sooty. This is considered undesirable in many breeds, as it gives a muddy appearance, but does not otherwise affect the dog. Some tanpoints have black 'thumbmarks' on each foot. This is also known as pencilling.

Sometimes the dog has more tan than usual - perhaps it has bled out on the face so that the entire muzzle and eye area is tan, perhaps most of the legs are tan too... This is what is sometimes called creeping tan (It can also be called for instance high or running tan). This appears as a fault in many breeds, while it is fully accepted in others (especially hound and some terrier breeds).

If the tan continues to creep, the tan will eventually be everywhere except on the back and perhaps tail and/or base of the head and/or ears. Then it would be called a saddled dog. Saddle-marked dogs look like regular black and tans as puppies and go through the creeping tan stages until they are adult and saddle-marked.

The genetics that give creeping and saddled tan, rather than the traditional markings, is currently not known.

Rottweiler with traditional tan points

An english toy terrier, with traditional tan points. This dog has clearly visible "thumbmarks" on each foot.

A crossbreed showing creeping tan points, also known as f.ex. high or running tan in some breeds. The pattern is somewhere inbetween traditional and saddle. This particular dog also has a black mask, which is an unrelated gene, but common in many dogs.

A welsh terrier in the saddled tan pattern. Saddled tan has sometimes been considered a completely separate pattern, but this author believes it belongs with the other tan point patterns.

Go here to see photo examples of a dog developing in the saddle pattern with age.