What is the best type of light for viewing printed photos?
The ISO standard, as explained in this document produced by X-Rite, a company that produces hardware and software used for color calibration, is to view prints in light that is at D50 (full spectrum centered at 5,000K) in terms of color temperature. In terms of intensity, around 2,000 lux (roughly equivalent to an overcast day) should be used for color decisions and judging (2000lux +/-500lux is within the standard, but +/- 250 is preferred), but intensity levels as low as around 800 lux may need to be used to observe fine differences in tonal qualities. The ISO standard also recommends at least 500 lux in a display setting, such as an art gallery. The light should be even, with the darkest areas at least 60% of the nominal value. The print should be surrounded by matting that is neutral in color and between 10-60% in terms of luminous reflectivity (i.e. Munsell N8/gray). And the light source, image, and viewer should be arranged so that there is no glare reflected from the surface of the photo to the viewer.
Barring access to a photo viewing booth specifically designed to meet the standard, a location that uses diffused daylight of sufficient intensity to provide similar lighting conditions is recommended, and don't forget to view the print against a neutral background.
It is really an artistic choice. The standard environment for lab color is as Michael Clark described, but display of a print and the gallery conditions can also be altered to whatever conditions you think best fit the print. Paper selection, light color or colors, framing and matte options, angle of display, size of display, printing technique, color, size and shape of the room are all factors that you can adjust and there isn't a right or wrong way to do it.
When choosing a lighting and display setup, you want it to feel natural with the image, but generally you will still want it to allow for colors to appear natural. This may also mean adjusting the color tone prior to printing to suit the color of the environment you will be displaying it in. If for example, the light will be very orange in color, you might reduce the amount of orange in the image and tint the white to be more blueish so that it will appear white when viewed in the gallery.
Typically, for reviewing images on screen, a relatively dark environment is suggested. Not so dark that the screen is overly bright, but dark enough that you get good contrast and the brights are vibrant. Color temperature is typically around 5k or 5.2k, but you can adjust it as you desire.
Basically any light that gives a full spectrum of colors, like daylight or incandescent light. Daylight may have a slightly yellowish tone, and incandescent light have a quite strong yellowish tint, but our eyes compensate for this, so that is only a problem if you are doing color correction so you have to compare the printed image to a computer screen.
Some flourescent light developed specially for this purpose gives a rather complete spectrum, but a regular flourescent light emits only a few colors from the spectrum. There is no way for our eyes to compensate for colors that are completely missing.