Shadeless

Rendering an object with a Shadeless material essentially allows the full color of the color or texture to visible, without any interaction with the rest of the lighting in the scene.   This is most often used in cases of background images, and with creating color masks within a scene.

In the Blender Internal this is done quite simply by checking the Shadeless box in the Material tool palette (as seen below).

In Cycles, it is a little more cumbersome, and the node setup (below) must be used.  Essentially what's going on here is that the material is set to Emmissive, but to prevent it from lighting other objects in the scene, the light paths are isolated to only casting toward the camera.