Internet Relay Chat (IRC) is a system that facilitates transfer of messages in the form of text. The chat process works on a client/server model of networking.
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Pro Open protocol
The IRC protocol is public and open, it is mature and stable, and has been implemented in numerous projects. This means that it can be used freely, for both open-source and commercial projects.
Pro Hundreds of avaliable clients
IRC has been around since 1988, so there is a ton of existing software to work with it. Everything from clients, to chat bots, to bouncers is already built.
Pro IRC networks are not centralized
You can choose your favorite server for an IRC network.
IRC offers choice. Choice in clients (mobile, desktop, web), in scripts those clients run, in servers and features those servers have. You can even set up your own server.
Every notable IRC client supports mentions - notifying you when your name is mentioned in the chat.
Pro Great historic importance
Twitter hashtags came from IRC channel names.
Pro No account necessary
No need to sign up for access, confirm your e-mail address. Just choose a nickname and you're done.
Pro Less distracting
Compared to other common solutions, IRC can be ignored when you are occupied doing something else. Most clients don't pester you with sounds or distracting visuals.
Pro Upcoming IRCv3 will provide missing features
Con Poor multimedia support
Sending anything besides text is not easy or intuitive and depends on the client.
Con Complex and not user friendly
Con Advanced functionality is client-dependant
Con Assumes some level of prior knowledge
The features are not discoverable the way they are in other apps and services.
Con You have to "idle" to see what's going on
You will see a blank chat screen at first because there is no recent history kept on servers.