A domain name is composed of name and two or three-letter extension separated by a period. The extension could be .com, .net, .org, .edu, .fr, .uk, etc. There is an extension for every country but .com is most common.
A domain refers to a device on the Internet. This device will be identified to your computer by its IP address. An IP address has a numerical format such as 18.104.22.168 since these are not handy for people to use or remember they are translated into domain names by the DNS (domain name server)