Each Ethereum network has a unique identifier that numerically represents the network chosen so that Ethereum clients and frameworks such as Truffle can quickly select a new testnet network. Ropsten, identified by the ID 3, is the name of the most used test network in Ethereum, because it provides the most similar technology stack to the real Mainnet, which is used by real-world dApps.
Notice that each testnet is a separate blockchain with its own set of rules and limitations to help people decide on the place to test their dApps, simulating real-world situations.
Initially, the Ropsten blockchain was named Morden and it was deployed right when Ethereum launched in 2015. After a year, the core Ethereum team decided to rename Morden to Ropsten to indicate that it was an upgraded version with better security features and faster performing...