Yahoo(TM) is a popular instant messaging protocol from the ever-popular 'Yahoo!'(TM) search engine and web portal. The IM protocol is often abbreviated to 'Y!'.
Yahoo is tightly integrated with their online portal. Users have the same IM address as their email address. It also gives you the option of seeing (through their website) if someone is online at any given moment, and contains (optionally) that user's public profile.
This protocol is implemented in Miranda through the Yahoo Plugin.