IPC (Inter Process Communications) generally works on you local computers. In the Ethereum space, IPC normally involves geth creating a IPC pipe (which is represented by the file $HOME/.ethereum/geth.ipc) on your computer's local file system.
Other processes on the same computer can then use the IPC file to create bidirectional communications with geth.
RPC (Remote Procedure Calls) generally works across different computers. In the Ethereum space, RPC normally refers to the RPC endpoint localhost:8545 or 127.0.0.1:8545 or 192.168.1.123:8545.
If you use localhost:8545 or 127.0.0.1:8545 for your RPC endpoint, other process ONLY on the local computer can communicate via this RPC endpoint, as localhost and 127.0.0.1 is only accessible from the local computer.
If you use a non-local IP address like 192.168.1.123, any other computer on your network can access this RPC endpoint.