Here's a good example of Panhard bar, it the black bar that on one end attaches to the rear end housing and at the other end attaches to the chassis, it's the black angular bar, where it crosses the center of the chassis, in our cars, the the center of the rear end is a good reference point, that will set the roll center, the lower the roll center typically the better a sports or road racing car will handle.
The second picture is on what is problably the world's most expensive Bugeye race car, it was Huffaker built and has a horizontal Watts link, I got to drive this car in four races in 2005, it's the orange car pictured in my avatar
Both the panhard bar and the watts link do the same thing they control lateral movement of the rear end in high g-forces, this is why many guys autocrossing go to a panhard bar, side laods, in normal street driving it's not really necessary, but as side loads increase with racing, you need a lateral support device. The panhard bar is one of my favorite designs because it require very little sub structure to mount, that's one of the downfalls to the horizontal Watts link in my opinion, Mumford link same deal, way too much stuff needed, and at the end of the day you don't want any more weight than you have to.