Ordinary Tor connection
Tor Bridge is the first server in the common chain of 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 a special one, which is impossible or very difficult to block.
Several types of pluggable transports can be used with bridges: obfs4, obfs3, scramblesuit, meek_lite, and snowflake. They use different methods of circumventing censorship.
Vanilla bridges do not use pluggable transport at all, but like other bridges, they use servers 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.