All nodes are important, but they have different technical requirements and legal implications.
Understanding the different kinds of nodes is the first step to learning which one is right for you.
Guard and middle relay
(also known as non-exit relays)
A guard relay is the first relay in the chain of 3 relays building a Tor circuit.
A middle relay is neither a guard nor an exit, but acts as the second hop between the two.
To become a guard, a relay has to be stable and fast (at least 2MByte/s) otherwise it will remain a middle relay.
Guard and middle relays usually do not receive abuse complaints.
All relays will be listed in the public list of Tor relays, so may be blocked by certain services that don't understand how Tor works or deliberately want to censor Tor users.
If you are running a relay from home and have one static IP, you may want to consider running a bridge instead so that your non-Tor traffic doesn't get blocked as though it's coming from Tor.
If you have a dynamic IP address or multiple static IPs, this isn't as much of an issue.
A non-exit Tor relay requires minimal maintenance efforts and bandwidth usage can be highly customized in the tor configuration (will be covered in more detail later in this guide).
The so called "exit policy" of the relay decides if it is a relay allowing clients to exit or not.
A non-exit relay does not allow exiting in its exit policy.
The exit relay is the final relay in a Tor circuit, the one that sends traffic out its destination.
The services Tor clients are connecting to (website, chat service, email provider, etc) will see the IP address of the exit relay instead of their real IP address of the Tor user.
Exit relays have the greatest legal exposure and liability of all the relays.
For example, if a user downloads copyrighted material while using your exit relay, you the operator may receive a DMCA notice.
Any abuse complaints about the exit will go directly to you (via your hoster, depending on the WHOIS records).
Generally, most complaints can be handled pretty easily through template letters, which we'll discuss more in legal considerations section.
Because of the legal exposure that comes with running an exit relay, you should not run a Tor exit relay from your home.
Ideal exit relay operators are affiliated with some institution, like a university, a library, a hackerspace or a privacy related organization.
An institution can not only provide greater bandwidth for the exit, but is better positioned to handle abuse complaints or the rare law enforcement inquiry.
If you are considering running an exit relay, please read the section on legal considerations for exit relay operators.
The design of the Tor network means that the IP address of Tor relays is public.
However, one of the ways Tor can be blocked by governments or ISPs is by blacklisting the IP addresses of these public Tor nodes.
Tor bridges are nodes in the network that are not listed in the public Tor directory, which make it harder for ISPs and governments to block them.
Bridges are useful for Tor users under oppressive regimes or for people who want an extra layer of security because they're worried somebody will recognize that they are contacting a public Tor relay IP address.
Several countries, including China and Iran, have found ways to detect and block connections to Tor bridges.
Pluggable transports, a special kind of bridge, address this by adding an additional layer of obfuscation.
Bridges are relatively easy, low-risk and low bandwidth Tor nodes to operate, but they have a big impact on users.
A bridge isn't likely to receive any abuse complaints, and since bridges are not listed in the public consensus, they are unlikely to be blocked by popular services.
Bridges are a great option if you can only run a Tor node from your home network, have only one static IP, and don't have a huge amount of bandwidth to donate -- we recommend giving your bridge at least 1 Mbit/sec.