Pixel art is a form of digital art, created through the use of raster graphics software, where images are edited on the pixel level. Graphics in most old (or relatively limited) computer and video games, graphing calculator games, and many mobile phone games are mostly pixel art. (source WikiPedia)


More info on (isometric) Pixel art:
Pixel Art http://en.wikipedia.org/wiki/Pixel_art
Isometric graphics by Zoggles http://www.zoggles.co.uk/asp/tutorials.asp?tut=1
Complete guide by Rhysd http://www.rhysd.com/tutorial/
PixelFreak's tutorial: http://www.19.5degs.com/element/869.php
Gas13 - Settingup Photoshop for PxelArt http://gas13.ru/v3/tutorials/isometric_pixelart_tutorial_setup_photoshop_for_pixelart.php