Availability Groups are a mechanism for setting up the ability to fail a database or a set of databases from one server to another. With this setup, there is nothing shared between the servers. They each have their own disks, their own CPU, etc. A cluster is at the server level. In a cluster, there is a shared disk between the two (or more) machines that each has to have access to in order for the service to work. You can get a lot more detail if you go and read the entries in the Books Online at MSDN.