Pixel Art

Pixel art is a form of digital art, created on the computer through the use of raster graphics software, where images are edited on the pixel level.
Graphics in most old computer and video games or even in Game Boy games are considered pixel art.
'Purists' believe that pixel art should be drawn pixel-by-pixel.
This means that if you use special tools to draw or create pixels for you, you are not creating pure pixel art.
Pixel art usually has a limited number of colors (often 256 at most) because of its traditions as well as simplicity.
Pixel art should never be saved in the JPG format because that is a lossy format that modifies pixels for you. You should save it as GIF or other non-lossy or pixel-modifying formats.
There are two main categories of pixel art:

Isometric Pixel Art

The art is usually seen from an approximately 25 - 30 degree angle.
This gives is a 3-dimensional effect. Games such as Sim City used this to better show off the world within the game.

Take a look at the following flash animation for an example of an isometric pixel art piece

Sim City in-game screen shot

Non-isometric Pixel Art

All other types of pixel art fall into this category.
This would include art that has a top-down view, side-view, etc.Games such as Super Mario and Bomberman fall into this category.

Human Body

Even though Mario evolved, it still remained pixelated

Many pixel-artists provide body bases for others to create pixel people from

Real-life imitations

Toys from Pixel Blocks