What is vector-based illustration?

All computer images are either pixel-based (as in photos you work with in Photoshop) or vector-based.

Pixel-based images (often referred to as raster or bitmap images) are made of tiny blocks of color (pixels), created at a certain resolution (a specific number of pixels per inch). The resolution for an image that’s destined for high quality output, such as a commercial printing press, is usually set to 300 pixels per inch (300 ppi). The resolution for an image that’s destined for low-quality output, such as art on a web page, is usually set to 72 ppi (a common screen resolution).

When you enlarge a raster image, individual pixels are enlarged, creating pixelated, blocky edges in place of smooth edges. If you enlarge a raster image too much, the image quality degrades noticeably.