From the zephyr man page:
Zephyr is a notice transport and delivery system developed at MIT.

Zephyr is an instant messaging system with several components.

zephyrd is the server application which maintains a database of subscriptions for every user. Redundant servers maintain duplicates of the database in the event of failure.

zhm (Zephyr Host Manager) is the application that runs on each end user's machine, and is responsible for communication between the end user applications and zephyrd.

zwgc (Zephyr WindowGram Client) is the end-user application which receives notices the user has subcribed to, and displays them. By default, each message is displayed in its own X window, as specified in the .zwgc.desc file in the user's home directory. Messages can also be handed to another client application, such as vt, or via a gateway to IRC or jabber.

zctl (Zephyr Control) manages the addition and deletion of temporary and permanent subscriptions, as well as managing other zephyr variables.

zwrite is the enuser application for sending messages to the zephyr servers, for subsequent retransmission to subscribed users.

Each message is sent to a triplet <class,instance,recipient>. Thr recipient is either "*" (sent to anyone subscribed to the class and instance) or a specific user (these users may be Kerberos principals), who will recieve it if they are currently subscribed to the class and instance specified. Users may subscribe to the wildcard instance ("*") for a given class, which will subscribe them to all instances on that class; however, they may not subscribe to the wildcard class.
Note that traditionally, messages to indivicual users are sent to class MESSAGE, instance PERSONAL, recipient user@realm. Class and instance subscriptions are not case sensitive.

Messages also contain an opcode, which is canonically used to send "ping" messages to check for a recipient's subscriptions prior to message composition. Opcodes have also been used to:

  • Specify PGP encryption
  • specify complete recipient list
  • specify original sender when using a gateway

Zephyr is in widespread use at MIT, CMU, Duke, and companies that are spawned by their alumni/ae.