This is the Skype API Plugin for Pidgin/libpurple/Adium.
The third-party Skype Plugin allows Adium to use the Skype API to display your Skype contacts within Adium and let you chat with them via text-based instant messaging. By the nature of the Skype API, it requires that the Skype program be installed and running.
It's not supported natively within Adium (i.e., without a plugin). The library that we use to support most protocols, libpurple, doesn't support Skype at all right now, not even for chat. Until it does Adium most likely won't, either.
Keep in mind that Skype is a proprietary, closed protocol, so it must be reverse-engineered before third-party clients like Pidgin and Adium can use it. Protocols like OSCAR (AIM) and MSN have already been reverse-engineered, but they're much older than Skype, and reverse-engineering is hard. It will take a long time and won't happen at all unless someone chooses to take interest in it and dedicate significant time to the effort.