In the development of fiber optic technology over the last 30 years, many companies and individuals have invented the "better mousetrap" - a fiber optic connector that was lower loss, lower cost, easier to terminate or solved some other perceived problem. In all, about 100 fiber optic connectors have been introduced to the marketplace, but only a few represent the majority of the market. Here is a rundown of the connectors that have been the leaders of the industry.

Check out the "spotters guide" below and you will see the most common fiber optic connectors. (All the photos are to the same scale, so you can get an idea of the relative size of these connectors.)

If you wonder what the connector names mean, see Fiber Optic Connector Terminology in The FOA Online Reference Guide to Fiber Optics. 

ST

ST (an AT&T Trademark) is probably still the most popular connector for multimode networks (ca. 2005), like most buildings and campuses. It has a bayonet mount and a long cylindrical 2.5 mm ceramic (usually) or polymer ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. A mating adapter is used to mate two connectors (shown below.) And because STs are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference.

The ST/SC/FC/FDDI/ESON connectors have the same ferrule size - 2.5 mm or about 0.1 inch - so they can be mixed and matched to each other using hybrid mating adapters. This makes it convenient to test, since you can have a set of multimode reference test cables with ST or SC connectors and adapt to all these connectors. See below.
 
SC

SC is a snap-in connector also with a 2.5 mm ferrule that is widely used for it's excellent performance. It was the connector standardized in TIA-568-A, but was not widely used at first because it was twice as expensive as a ST. Now it's only a bit more expensive and much more common It's a snap-in connector that latches with a simple push-pull motion. It is also available in a duplex configuration.
  
FC

FC was one of the most popular singlemode connectors for many years. It also uses a 2.5 mm ferrule, but some of the early ones use ceramic inside stainless steel ferrules. It screws on firmly, but you must make sure you have the key aligned in the slot properly before tightening. It's been mostly replaced by SCs and LCs.