I would define like this: Computer network is a sum of technologies which enables to share data between devices connected with wires, optic cables, WiFi or any other physical technology. In computer terminology connected devices called as nodes, which can be any device that has network adapter such as computers, micro controllers, smart phones, smart TVs.. etc.
There is 3 types of computer network - LAN (local area network), MAN (metropolitan area network) and WAN (wide area network). As an example of LAN - we can say networks inside your house, work or school. MAN - it's a network for a city or metropolitan or region (it depends on the size of country/city). WAN - is called any network bigger than MAN, the most popular one is Internet.
Internet is one of Computer networks and it's the biggest one. In the internet clients can talk to other clients, as well as they can talk to servers using many protocols. The most important one TCP/IP, UDP, SSH, FTP, HTTP, SMTP etc.
Here we should note that protocols form separate layers, TCP and UDP protocols are in Transport Layer which is used by Application Layer like HTTP, SSH, FTP etc.. So Application layer is built on Transport Layer (TCP and UDP). Each application layer protocols has it's own port, for example HTTPS is using 80 TCP port, FTP is using 22 TCP port, HTTPS is using 443 TCP port ..etc.