The domain addresses are basically, the names we give to websites. For example the domain address of Code+Marketing is codeplusmarketing.com.
The domain addresses are formed thanks to the Domain Name System or DNS for short. The DNS system allows the distribution of addresses from the web servers. Just like the telephone numbers, the domain system allows the assignment of easy to remember addresses to an IP address of a given server. The IP address has a typical structure of combination of numbers, something like 22.214.171.124. If we make an analogy with the mobile phone numbers, then you can think of the IP address like the telephone numbers of the people in your contact list and you can think of the domain address like the names of the people in the contact list. We should point out that the domain address is unique. In other words, there cannot be two completely identical domain addresses. But two different domain addresses can point to the same IP address. For example abc.com and xyz.com can point to the same server (IP address). Also it is possible to change a domain address corresponding to a given IP address.
The domain addresses are organized in the DNS root system. At the top are the so called Top-Level Domains or TLD. There are different variations TLDs like country TLDs (ccTLD), generic TLD (gTLD) etc. In short, the TLD part of a domain address is the .com part. Popular TLD include .com, .net. co.uk. Recently ICANN, the organization governing the TLDs, made possible the creation of new ones. Now there are top-level domains like .barcelona, .la .boutique .auto etc. In theory, TLDs are available for reservation/purchase from anybody as long as the TLD is free and the company/person who purchases it pays all the fees (the documentation fee starts from 180 000$). In reality however, most of these top-level domains are reserved from big companies and resellers.
The other major part of the domain address is the so called subdomain. In codeplusmarketing.com, this is the codeplusmarketing part. These subdomains can be purchased by anybody, as long as they are not already reserved. You should keep in mind that the whole address consists of the subdomain + TLD. This means that auto.com may be reserved but auto.net may be free, and you can only purchase the one with the .net TLD. Typically, the domain addresses are purchased for a period of time and their prices vary according to different factors. For ordinary domain addresses with .com TLD, it is around 20$ per year.
Some advices for choosing the perfect domain
How can I buy a domain?
Well, you can simply google “buy domains” and you will get lots of results. The reality is that there are plenty of domain providers (or Registrars as they are called) and each of them offers a different experience. Some of them offer exceptional service/support but their prices are higher. Others follow the discount model. Keep in mind that exotic top-level domains may not be offered by the registrar of your choice. Some hosting providers also offer domains.