Bitmap (Pixel) Vs. Vector

Basically a Bitmap illustration is a picture that is made up of many little squares to make up a big picture.

    

Because is based on squares when you expand a bitmap image the quality of the picture is not that good and is actually a little blurry. 

Each Vector image is made up of smaller pictures like the bitmap but the pictures in the Vector image can be scalable.  If you resize vector images they wont loose the quality of the picture.