A fiber optic connector is a device used to plug a fiber-optic cable into an electronic device. It is attached to the end of a fiber-optic cable and designed to keep the fibers in place so that they can be properly aligned with the fibers on the other end of the connection.
There are many different types of fiber optic connectors, of which the more common is the SC connector, LC connector and FC connector. SC connector has a push-pull locking mechanism which is very flexible yet provides high repeatability and low insertion loss. It has been quickly replacing legacy connectors such as ST, SMA connectors and becoming the current most popular fiber connectors used in the fiber optic communication industry. The SC connector may be coupled in duplex sets with a coupling receptacle or duplex clip. In a keyed duplex set, the SC connector easily implements a form of polarity matching with coupling adapters in fiber optic outlets or patch panels.
LC connector is a small form-factor fiber connector which uses a 1.25mm diameter ferrule instead of the more legacy 2.5mm ferrule used on FC, ST, SMA and SC. LC is a high performance connector especially designed for single mode applications. Actually all LC connector are not created equal and depending on style and manufacturer's preference there may be attributes that make one connector more suitable for a specific application than another.
FC fiber optic connector is popular in test environments and mostly for single mode applications. FC connector has a threaded coupling mechanism with adjustable keying to achieve minimum insertion loss. You can adjust the key in some degree in order to lock it at the minimum loss position. Although this tuning is useful in some applications where minimum loss is required, it is not used in most applications because of the small gain of 0.1~0.2dB in insertion loss.