From what I understand, a testnet is just a local (or private) blockchain for developers to experiment on, and TestRPC is/was a specific implementation of a testnet for the Ethereum blockchain?
(I say 'was' because it appears Truffle absorbed TestRPC)
Testnet is a real network for test purpose similar to the mainnet but with worthless Ethers.
TestRpc or ganache is a blockchain simulator which runs 100% locally which is much much faster than a any other deployment environment making it ideal for testing and development.
