There are many different display technologies used in smartphones, tablets, and other ultramobile devices. The most common of these (in no particular order) are:
There are also several ways to lay out the subpixels in a display. The RGB stripe subpixel matrix is the most common, with each pixel containing one red, one green, and one blue dot.
PenTile subpixel matrices arrange the subpixels in a checkerboard pattern, which can offer different advantages depending on the implementation, but requires an even number of dots per pixel. Samsung's RG-BG implementation is probably the best known, with two subpixels per pixel. This allows Samsung to increase the effective pixel density by 33%, while retaining the full number of green subpixels. Because human vision peaks in the green portion of the visible spectrum and falls off towards red and blue, the red and blue subpixels are less important for overall smoothness. Nonetheless, it's still possible to notice the lack of red and blue subpixels, especially on thin, vertical, blue UI elements.
Motorola has used an RG-BW PenTile subpixel matrix on some of their devices in the past in order to maximize brightness, but its lack of sharpness is clearly apparent, and was slammed by reviewers. Sony's WhiteMagic technology is probably the best implementation of the PenTile subpixel matrix, with 4 dots per pixel (RGBW).