ST stands for Straight Tip - a quick release bayonet style Connector developed by AT&T. STs were predominant in the late 80s and early 90s.

Features:

Low insertion loss(≤ 0.20dB)and high return loss(≥ 55 dB)
Free-floating ceramic ferrule
UL-rated plastic housing and boot
Boots in a variety of colors
High precision alignment connector

Applications:

Local Area Networks (LANs) and Wide Area Networks (WANs)
Fiber Optic CATV, FTTH, FTTB, FTTP etc
Fiber Optic telecommunication systems
Transmission Mode (ATM)
Fiber Optic Backbone
Military Instrumentation