This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
To elaborate on Tim's answer, it is installed on both. The server typically contains the repository of data. After all you want this to be available even if someone is out. The client is installed on the client to connect to the repository.