The most basic design of snips are tin snips, which simply consist of two blades joined at a pivot point. Working in the same way scissors do, these blades are two levers which cross each other. These snips are used for cutting tough materials, like sheet metal and plastic.
Tin snips are generally quite heavy tools so are not really suited to continued use. The size of the tin snips will reflect the thickness of material they can cut, with larger tin snips being able to cut thicker and tougher materials.
There are many variations of tin snip design, for different uses and materials.