In a MUD, we start with a virtual world in which people log in remotely (usually via telnet), that has a basic text interface with which to communicate and compete with other users to gain experience points, treasure, and so on. Of course sometimes people get bored of fighting and sit and chat with their friends across such a system.
This is where the talker comes in. If we strip out all the combat and competition, and leave the communication parts, we have a very nice chat system. Most code bases are open source, so anyone can add miniature games and amusements to this system. Fans of talkers believe this gives a real feeling of mutual enjoyment in a nice atmosphere.
Alternatively, in a more modern context, we have the chat system. Typically, these are large systems with proprietary interfaces and limiting abilities, such as the chat rooms on AOL. So if we make a standard interface accessible via IP address and a standard port number across telnet, then we can make it so anyone can login in without much in the way of proprietary downloads. Then data all comes streaming across standard routes.
Talker servers can be run on almost any Unix system. Many telnet clients exist and one usually comes standard with the computer's operating system. Many users of talkers enjoy the use of clients such as GMud, zMUD, TinyFugue, the basic telnet interface in a UNIX system, or the standard Microsoft Windows telnet client.
People who use talkers are often referred to as spod.
Talkers tend to run on a number of code bases, such as ew-too, summink, sensi-summink, and playground.