The longstanding joke among surgeons is that the best surgical retractors are third year medical students. This joke, based on a sad daily reality, is that one of the primary responsibilities of a third year medical student during their surgical rotation is to retract. The student watches and learns about the surgery while holding organs or tissues out of the way for surgeons so that they can operate. In truth, the simplest and often most delicate surgical retractors are human hands (covered by sterile gloves, of course).
Surgical retractors are meant to keep the surgical field open and clear enough so that tissues of interest can be clearly seen and manipulated. In other words, in order to cut and suture things deep within the body you must first get to them and then be able to see them. Surgical retractors help do both. They also double as surgical graspers from time to time depending on the surgery and approach chosen by the surgeon.
There are seemingly limitless different types of surgical retractors. Virtually every one was named after the surgeon that first introduced it to their field of surgery. There are at least 35 different, named surgical retractors, each with a different shape, purpose and application.
Jokes aside, the gloved human hand is the simplest (and without the glove, oldest) type of surgical retractor. The next simplest retractor would be a simple foreceps. The end that is handled usually has finger and thumb holes like a pair of scissors so that the foreceps can be closed and pulled back if needed. The other end of the foreceps simply pinches the tissue that is being retracted. Simple forceps might be used to hold small objects.
For more delicate, but still small tissues, the jaws or business end of the surgical retractor might be curved or tapered. This shape is to prevent damage to the retracted tissue. This specialization is also how many of the dozens of different types of surgical retractors are differentiated.
For larger tissues, surgical retractors take on a different shape altogether. Instead of having holes for fingers, they are held with simple handles or hooks. The jaws are replaced by their own hook, usually larger. The hooks, called blades, are placed around larger tissues or whole organs so they can be gently moved and held out of the way. The blades may be curves, have right angles, or look like a whisk for beating eggs (except flat). The specific blade on the surgical retractor used is based on the size of the organ and how sturdy the covering on the organ. For example the lungs are less hearty than the liver, but the liver is less hearty than the kidneys. Lungs would only be touched by appropriately delicate surgical retractors while the kidneys could stand up to more rigorous traction.
The above surgical retractors need an assistant (like a third year medical student) to provide constant pressure on the tissue. One of the types of surgical retractors does not need constant pressure but rather holds itself in place. The so called self-retaining retractor can incorporate any number of different blades but what sets it apart from other types of surgical retractors is that can hold itself into a given conformation. A ratchet or friction glide is used to hold the blades in place once the surgeon moves them into the proper position. While the surgeon may need to reposition the self-retaining surgical retractor from time to time, the device usually frees up the assistant for other things, like watching quietly in the case of a third year medical student.