Oracle RAC is a shared-data architecture, so all nodes can be active at the same time, and all can be servicing requests from your applications. The RAC software takes care of the management and correctness of the data so that if one node changes the data in memory, then other nodes will still be able to give an accurate representation of the data. So all nodes are active participants.

Master/Slave is typically done at an OS level, so all activity is performed by a single node only (the master). The other node (the slave) sits "passive", basically doing nothing except watching the master to see if it is responding to requests. If it is seen to be offline or unavailable, the slave will typically "kill" the master, ie, block any access from the master to the storage and then take over the operations.

The two are not mutually exclusive. There are hybrid configurations where in a RAC cluster, certain types of work will only be performed by a subset of the nodes to minimize cross-node communication, or I've seen examples where RAC is in use, but is predominantly master/slave but having RAC makes the failover time extremely short etc.

So RAC can give you potentially better usage of all of your nodes, and more flexibility....but of course, you pay a license fee for that benefit.