A domain is purchased and registered with a domain registrar. It is a unique location on the web that includes a unique series of characters, and a TLD ending, such as .com, or .org. Referencing a domain, generally indicates that someone is talking about all the assets hosted at that domain, including all the pages and supporting resources like images and code. It also often references the supporting technology that is used to host that content, such as the server, and related utilities like CDNs.