|Photoshop is raster-based, as opposed to vector-based (Illustrator). This means it uses pixels, which have colors associated with each one.|
|Part of||Photoshop CS5|
Vector vs. Raster
To understand when it's best to use Illustrator, you need to get a handle on vector and raster images. A rasterized image is made up of pixels and does not scale up without decreasing quality, while a vector image contains is merely point information connected by specific curve angles and can be scaled up without any loss of quality.
In the image above: ' Vector: Left, Raster: Right