A portrait lens is one that is used for taking pictures, usually of the head and shoulders, of a person. It may differ from a normal lens in a number of ways.

1. Focal Length
The traditional 35mm full-frame lens for portrait work was between roughly 75mm to 105mm.
On a 1.6X APS sensor camera, this would be roughly 50mm to 70mm. While some people, I suspect ones with rather large noses ;) , like longer lenses, the majority of photographers over the years have found too long lenses to flatten the face too much, while shorter lenses tend to bring out the size of the schnoz and other such features. Most manufacturers make inexpensive 50mm primes that are still excellent in image quality (IQ), and these are very good for portraits on the smaller sensor camera.

2. Lens Sharpness
A second consideration in a portrait lens is its sharpness. If you are doing a Diane Arbus or a Farm Service Administration portrait (q.v.), then you may want to show every little wrinkle and blackhead. Otherwise, most portrait photographers who are hired to make portraits have come to treasure "soft" lenses that smooth out the face. Some lenses like the much lamented Spiratone Portragon were especially made to be soft portrait lenses. However, while it is difficult to make a soft picture sharp, it is always possible to make a sharp one softer either by using a diffusion filter at the time of taking the picture, or by using something like a Gaussian blur in an image-processing application.

3. The Mysterious "Bokeh"
A third factor in portraiture is what in Japanese is called 'bokeh'--the pleasantness of the out-of-focus portions of the photograph. Usually portrait artists have found it useful to accentuate the portrait by throwing the background (which can be distracting or disastrous, depending on what it is and whether it seems to put horns on the subject, for example) out of focus. This is more easily done if the lens has a wide aperture. The almost universally preferred bokeh is to have the background show up as circular blurs, not pentagonal, etc. This effect is caused by the lens diaphragm blades, so manufacturers have been making lenses with more circular appearing apertures.

Selecting a Lens

Often, photographers may want to combine portrait work with the ability to do macro photography. In selecting a lens for portrait use that will also serve for macro use, however, you encounter a contradiction (see Engels and Mao Tse-Tung). For true macro work(1:1, that is, the size on the film/sensor is the same size as the original), it is usually better to have longer focal lengths to increase the effective 'working distance' between the front elements of the lens and the subject. Thus a 90mm macro like the Tamron (a superb lens by almost all tests), will work like a 144mm telephoto on a 1.6X camera. This is not unheard of for a portrait lens, but many would consider it to be a little on the long side. The other problem with macro lenses for portraiture is that they are usually exceptionally sharp lenses, so a diffusion filter may be a good idea.

A solution for Canon users is the Canon 60mm EF-S macro lens. It is the equivalent of a good 90mm on a full-frame camera (right in the portrait range!). The reviews are very good, but if you're photographing shy subjects like live insects in macro mode, then you'd do better with a longer lens.

For Canon users, the plastic fantastic--the Canon EF 50mm 1.8 lens--is such a bargain, you ought to get one regardless! If you are looking for a Nikon lens, one of the sharpest ever made is the Nikkor-P 105mm F/2.5 lens and its descendants,  a little long for most Nikon digital cameras for portraits, however.

[This guide is based on an answer written by me on a Photo.net thread]