Intent: this pop-up menu allows the user to select from four different rendering intents, namely Perceptual, Saturation, Relative Colorimetric and Absolute Colorimetric. Typically, most users will choose between either Relative Colorimetric or Perceptual. A short description on each is provided in the Description section of the Color Settings dialog. A more comprehensive explanation can be found in the Photoshop on-line help files.
With Relative Colorimetric it is only those source colors that are out of gamut (i.e. can't be viewed/printed accurately within the destination color space) that will be mapped to the closest in-gamut color, the remainder are left unchanged. This means that in the case of documents containing lots of out-of-gamut colors the visual relationship between the colors (after conversion) will almost certainly change. With Perceptual, all colors of the source color space will be mapped to the nearest in-gamut color of the destination color space thus maintaining the visual relationship between colors. In other words, with Perceptual the whole document color gamut will be compressed so that it fits within the new color space. The Photoshop default is Relative Colorimetric.