What are hinges (located next to welds) and how do they work?
Put simply, hinges allow parts to move yet be connected to another part.
For example a door: A hinge at the top and the bottom means that the door can rotate about the hinge. Or maybe a wheel - a cylinder with a hinge connecting to the body of a vehicle means the wheel can rotate around the hinge.
They are most useful in those two scenarios - wheels and doors.
http://wiki.roblox.com/index.php?title=Brick_surface_types#Hinges
Hinges are basically connections between two parts that allows them to rotate and spin without interfering with each other. It creates a movable joint, or mechanism, that allows the parts or objects to move around. See more about Hinges here.