Tor Bridges
Tor Bridge is an entry Tor server. It is the first server in common chain of 3 Tor servers, which is required to protect your privacy. Like ordinary Tor servers, bridges are run by volunteers; unlike ordinary servers, however, they are not listed publicly, so an adversary cannot identify them easily.
Using bridges in combination with pluggable transports helps to disguise the fact that you are using Tor, but may slow down the connection compared to using ordinary Tor servers. Also in some countries ordinary Tor entry servers are blocked, and/or Tor network is blocked by DPI. But you can bypass blockage by using bridges with pluggable transport. Pluggable transport is designed to transform ordinary Tor traffic to special one, which is impossible or quite difficult to block.
There are few types of pluggable transports which can be used with bridges: obfs4, obfs3, scramblesuit, meek_lite and snowflake. They are used different techniques to bypass censorship.
Vanilla bridges do not use pluggable transport at all but, like other bridges, use entry server that are not listed publicly.
Obfs3 and scrablesuit bridges are deprecated.
Obfs4 bridges makes Tor traffic look random, and also prevents censors from finding bridges by Internet scanning.
Meek transports make it look like you are browsing a major web site instead of using Tor. meek_lite makes it look like you are using a Microsoft web site.
Snowflake sends your traffic through WebRTC, a peer-to-peer protocol with built-in NAT punching.
The most commonly used bridges are obfs4.