For the purpose of your question, "Internet" is your ability to connect. It's what you pay your service provider for. You have a modem somewhere in your home that the company sends internet to, and you have to connect your devices to that modem to get online.

Before wifi, you had to use wires to connect to the modem. You would an actual wire running from the back of your computer all the way to the modem.

Wifi is a wireless connection to your modem. Most modems these days are also "wireless routers" that turn your internet into a wireless signal. That wireless internet signal to your modem is wifi.

Your devices all have wifi transmitters that receive the wireless signal from the router/modem, and the modem then connects you to the internet.