About Image Types
Raster images (sometimes referred to as bitmap images) are made up of thousands of pixels that determine colour and form. Photos are raster images. Photoshop is the most common raster editor, enabling you to manipulate the colour and other properties of the pixels.
Because raster images are made up of a finite number of pixels, resizing can be tricky. If you give a raster image larger dimensions in Photoshop, the software must make up data to add the size. This results in a loss of quality.
Vector-based images (such as those created in Adobe Illustrator) are made up of points, each of which has a defined X and Y coordinate. These points join paths to form shapes, and inside these shapes, you can add colour fills. Because everything you generate is based around this, vectors can be blown up to any size without any loss of quality.