What Is the difference between the testnet and the mainnet? I have been seeing some questions about the testnet but what is the testnet?  Is the mainnet the one we normally use? What is the difference?
The testnet (test network) is an alternative Bitcoin blockchain, to be used for testing. Testnet coins are separate and distinct from actual bitcoins, and are never supposed to have any value. This allows application developers or bitcoin testers to experiment, without having to use real bitcoins or worrying about breaking the main bitcoin chain.

Mainnet: The original and main network for Bitcoin transactions, where satoshis have real economic value.

We're using the testnet alot for developing purposes, assume you are building a new feature and its require you to send and receive bitcoins and you don't have any real bitcoins or you don't want to waste with mistakes, here we use test network (testnet).

Testnet addresses and private keys have a different prefix 

Testnet BIP32 private key   tprv    tprv8ZgxMBicQKsPcsbCVeqqF1KVdH7gwDJbxbzpCxDUsoXHdb6SnTPY

Testnet BIP32 pubkey    tpub    tpubD6NzVbkrYhZ4WLczPJWReQycCJdd6YVWXubbVUFnJ5KgU5MDQrD9

Testnet pubkey hash (m or n)    mipcBbFg9gMiCh81Kj8tqqdgoZub1ZJRfn