What is the difference between filler panels and end panels?

A filler panel is a spacer panel used to fill a small gap or void in a run of kitchen units, as opposed to an end panel that is used on the side of a cabinet to match the doors, finishing the edge of a run of cabinets. 

What is a filler panel made from? 

A filler panel is usually made from either a door that is ripped (cut with a rip saw or plunge saw) to exactly the right width, or a piece of plinth cut in the same way. 

What is an end panel made from?

An end panel is a a matching panel, normally the same height of the cabinet including legs, or sometimes the same height as a kitchen door. This is 720mm high on base panels. The height of wall end panels depends on the height of the wall cabinet, usually 475mm, 720mm or 900mm high.