According to Wikipedia, BuddyPress is open-source social networking software package owned by Automattic that can be installed on WordPress to transform it into a social network platform. Perfect, I thought, for a client who organizes educational fairs.
BuddyPress includes its own theme, which looks like your average bulletin board from the late 1990s. Functional, but not pretty. Having never used BuddyPress, I was surprised when it activated and replaced my premium theme and had no idea I would have to shake off that built-in theme. Turns out, what I needed was, you guessed it, another plugin.