ST (an AT&T Trademark) was probably still most popular connector for multimode networks until 2005. It has a bayonet mount and a long cylindrical 2.5 mm ceramic (usually) ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. A mating adapter is used to mate two connectors. And because ST connectors 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.
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 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.
Mating Dissimilar Connectors
The ST, SC and FC connectors share a 2.5 mm ferrule design so they can be mated to each other. To do so requires a hybrid mating adapter.
From the top:
In the past, you could also get hybrid adapters for these connectors to FDDI and ESCON connectors which used the same ferrule.
LC is a small form factor connector that uses a 1.25 mm ferrule, half the size of the SC fiber patch cable. Otherwise, it's a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for singlemode.
The LC, MU and LX-5 use the same ferrule but cross-mating adapters are not easy to find.
MPO is a 12 fiber connector for ribbon cable, often called an array connector. The connector ferrule is plastic and contains 12 or 16 fibers in a row. The versions with 12 fiber rows can theoretically have 6 rows but more than two are rare. The 16 fiber rows are limited to 2 rows. Fiber ferrule alignment is by pins in one ferrule and holes in the other.
The MPO main use is for preterminated cable assemblies and cabling systems. Here is a 12 fiber MT broken out into 12 STs. It is also used for higher speed multimode links (40 or 100Gb/s) that use parallel optics.
MPO means multi-position optical. This connector is sometimes called a MTP cable which is a commercial name. It is covered in the TIA connector intermateability standard(TIA-604-5) for the 12 fiber version and OEMfibers for the 16 fiber version..
For more information on the MPO connector, including its issues with polarity and testing, go here