The parallel arm saw is probably the most common type of saw on the market. There is a top arm and a bottom arm that are connected by a link at the rear. Each the arms pivots on it's own pivot bolt that allows the blade to travel in a close to vertical position.

Exaggerated  drawing of the movement of the arms . Notice that the blade maintains a mostly vertical position during the full up and down stroke, as the arms are being driven.

The Delta Q3 and the Craftsman Contractor are the C-arm design. There is no link as the top, bottom arm, and link are all one solid unit. This causes the rocking motion to the blade travel path.

Since the c-arm pivots only from the rear of the arm the blade is swinging in an arc. This is what cause the undercut that you hear about from a c-arm. But the undercut is really exaggerated to be more than it really is.

The Dewalt and Excalibar are a design of their own. This is called a parallel link arm type saw. The blade moves in a more constant vertical stroke, but does move front to rear in the process.

There are pivots at the reak on the link, and then pivots at the front of each arm on the swinging triangle frame. This makes this saw the one with the most moving parts to the arms.