Tor: What is the difference between a relay, bridge, and exit relay?

Short answer

A relay is a server that carries Tor traffic. A bridge is an unpublished entry point used mainly to help people bypass blocking; a middle relay carries traffic between other relays; an exit relay makes the final connection to an ordinary website. These roles have different operational and abuse-report considerations, so choose one deliberately before offering a server.

Sources

Tor Project: Relay types.