Rendering intents are components of the ICC profile. They allow the user some options in how color transforms, based on different intentions or uses. Because different image input and output instruments have different physical properties, it is not possible to expect good color reproduction to occur merely by always reducing color from one color space to the nearest point on another space. The four rendering intents are:
Absolute Colorimetric tries to reproduce colors scaled to the white point of the source color space. This is mainly used when proofing colors. In other words, use Abs Col when trying to get one printer to look like another printer - including the darker white of the original.
Relative Colorimetric tries to reproduce colors with as little desaturation as possible. Out of gamut colors are moved to the nearest point within gamut. Unlike Abs Col, colors are all scaled to the white point of the destination color space.
With Perceptual, colors are generally desaturated in order to preserve their relationship. This can make for very smooth transitions between colors. This avoids the banding that is possible with Rel Col, but at the sacrifice of some color saturation.
The Saturation rendering intent is similar to Perceptual, except that more saturation is preserved at the expense of hue accuracy.
The most commonly referenced rendering intents are Relative Colorimetric and Perceptual.
Anatomy of a Profile (ColorNews Article)