A domain is the human-readable address of the website. Computers communicate via numbers called IP addresses. Domain names mask those numbers/ IP addresses into something that people will easily remember and understand.
Each domain name is unique and consists of a top level and second level domain. The top-level part is located right after the dot. The most popular top-level domains are .com; .net; .org; .biz. A person can chose which top-level domain to register according to the type of website they are planning to build, or already has.
The second level domain is the part of the domain that is before the dot.