Domain Name

What is a domain?
Essentially it's your web address where people can find your web site. Without a doman name your web address would be a string of numbers or IP, the IP of this site is 74.119.144.50, so a domain makes it much easier for people to remember. There are many extensions but the more popular are .com, .net and .org just to name a few. The more common extensions range from $10 - $15 but some extentions can be much more. These are also known as a TLD or Top Level Domain and you will need one if you plan on having your web site hosted by a paid host. When picking a domain be prepared on having a list of alternative options other than your top choice as often times the name is already taken unless you are using your personal name or have a very unique business name. You'd be surprised as to how many domain names are already taken.
What is a Subdomain?
Subdomains are different and look different. An example of a subdomain of this site would look like this: blog.9wired.com. Even though a subdomain is part of of the TLD it acts and performs just as if it were its own web site and is treated as such by search engines like google. Free web sites will often let you use a subdomain of their domain instead of requiring a TLD but most have the option of using a TLD for an extra charge.
Registrar
There are many registrars to choose from and most web hosts also sell domains but it is often recommended that you purchase your domain separately from your web hosting. In the early days of the web there were many horror stories of web hosts holding domain names hostage but you don't here much of it these days, still most experts recommend against it.
Buying a Domain
When buying a domain many webmasters will buy several extensions of the TLD but this can get quite costly. Unless your an organization or some other entity that a certain extension fits for you, I recommend buying the .com and .net for your particular domain. The reason I say this is that you don't want someone else to buy an extension of your domain and cause confusion for people looking for your web site. This may be confusing in itself. Let me see if I can clear this up for you. Say you want myname.com, someone else could buy myname.net and then your followers would be confused as to which site is yours. Usually most people will stay away from a particular name if the .com is already taken but I've seen a .com and .net of the same name owned by two different people, you really want to avoid that confusion. That's why I say to buy both and that will likely keep others from buying a different extension of the same name. I hope I made this clearer than mud.
Namecheap
I personally own seven domains at this time and five of them reside at namecheap.com. I originally chose them because of their easy to use control panel. Recently they redesigned their control panel and unfortunately, in my opinion they made it much more confusing and complicated but they have top notch support so it sort of equals out the confusion, even their live chat support is very helpful and knowledgeable. I still recommend namecheap for anyone looking to buy a domain.
Domains Defined
You may have heard of parked and addon domains and wondered what exactly they are and what's the difference.
Parked Domain
A parked domain is another domain name owned by you that points toward your main domain's web site. For example, if your main site is mysite.com you could also have mysite.net pointing to your main site as well. This is useful if someone types in .net instead of .com, they would arrive at your site anyway.
Addon Domain
An addon domain is a domain owned by you and hosted on the same cPanel account as your main site. Addon domains act as their own site, searchable by their own domain, except they do not have their own separate cPanel. All of your addon domains would share the same cPanel. You may see shared hosting plans advertising to 'host 10 domains', this means you could host 9 addon domains in addition to your main domain.

Updated 4-3-16