A bitmap (e.g. JPEG, PNG, GIF) is made up of a thousands of tiny squares or 'pixels'. These pixels are all the same size, but can be in a huge range of colours. The amount of pixels shown in an image is called 'resolution'. When there are lots of pixels and an image looks smooth or photographic, that's 'high resolution'. When there are less pixels an image might look blocky or 'pixellated'.
Due to the fact that there are a set number of pixels in a bitmap image, they don't hold up well when zoomed in or enlarged. (You can test this yourself, by zooming right into one of your own images on screen, it will look blocky the more you zoom in and less pixels are available.)
For this reason, we ask for 'high resolution JPEGS' - these have a large number of pixels available and will look smoother when printed.
A vector image is more sophisticated: it uses X and Y coordinates to plot each point on a line or curve. This means that vector images are scalable and can be enlarged to billboard size while maintaining smooth edges.
Where possible, we recommend saving graphic designs, text and line art as 'vector based' PDFs. This is possible in applications like Adobe Illustrator, Adobe InDesign and more recent version of Adobe Photoshop. You can download templates for each of these applications.