That depends on what application you are clustering. Assuming Windows, if you run Exchange, it is recommended to do active-passive clustering. In AP clustering, you can have up to 8 nodes in the cluster, and up to 7 seven virtual instances of Exchange (called EVS, exchange virtual server). So you have 7 nodes, each running a separate EVS, and one passive node. If any single node fails, the passive node will assume responsibility for that failed node's EVS. If a second node fails, that node's EVS will remain offline until a node becomes available.
In the active-active Exchange scenario, you are limited to two nodes, and two EVSs. In this case, each node is running a separate EVS. If one node fails, the other node will run both EVSs. However, if the second node fails before the first node is restored, you have nothing.
If you want to learn about Windows clustering, start here:
http://www.microsoft.com/windowsserver2003/technologies/clustering/default.mspx