Domains are the human-readable translation of an IP address. So, what's an IP address? A computer-understandable series of numbers, that essentially tells one computer how to find another. The internet, of course, is a series of inter- connected, constantly communicating computers, passing information between one another. Even a server is a computer, the only difference between a server computer and your laptop, is that the server computer is designed to be accessed by users coming from the outside, and has a host of special apps installed on it to help with this, whereas your laptop is shielded from outside access, and lacks these programmes.
So, to communicate with another computer, your computer needs to know where to find it. IP addresses are called addresses because they function much like your home address, or wherever you get physical mail at. But you wouldn't very easily be able to remember these numbers to access sites you frequent. You'd have to type in 188.8.131.52 every time you wanted to get on Facebook, and this IP address actually varies depending on where a user is located: Facebook has at least 50,000 different servers, and depending on where these servers are, each server location has different IP addresses.
This is called a server block, or a server cluster, wherein a large site spreads its information and access load between multiple server computers, all accessing their own information from the same general data pool, to prevent the website's massive visitor numbers from taking the server offline by overloading it. This also enables the server block to choose a location for the user automatically, so that they get the closest server to them, and the fastest speed. So, instead of having to type in 184.108.40.206 to access Facebook, you just type in facebook.com, and DNS servers decide where you want to go.
Domains can come in many types: top-level domains have only one period. I.E. google.com, facebook.com, wikipedia.org. A top- level domain is usually rather sought after for businesses and web-admins that are concerned about their search engine rankings (how high on the search list their site appears for keywords relevant to their site), because top-level domains tend to rank better. Sub-domains have two periods. I.E. fonts.google.com, analytics.google.com, en.wikipedia.org, m.facebook.com. These are typically suspected to be more specialised or descriptive than the top-level. For instance, Google's Web Fonts are available on fonts.google.com, while its mail service is available on mail.google.com. This attaches it under Google's main branding, but also separates the services into more specialised categories. Finally, there are directory domains. These are URLs that are attached to a specific folder within a top-level domain's file server, or served through CGI (oftentimes there are commands in the URL for CGI to interpret and make sense of). These URLs look like domain.com/site.
IP Tech has two domains to offer: inpopul.us, or kazeshini.net. Either one can handle subdomains and directories. However, both of the paid hosting plans offer a top-level domain of your own for free (you will lose it if you drop hosting).