When you first start out creating a website, it can be a bit confusing with all the different things you need to know. One of these is what a domain is. (Note that the term "domain" is short for "domain name" and are both used interchangeably.)
A domain or domain name is the address of the website. The domain of this website is "imlearningjoomla.com". Or kind of... there's a bit more to it.
Domain Name Extensions
If the whole of the internet is a country, the next level down would be states (if you live in a country with states). In the country of the internet, states would be domain name extensions (such as .com vs .net vs .org vs .info vs ...).
Below domain name extensions (the states of the country) are the domain names, which would be the cities and towns. The domain name is the part that comes before the .com or .net. In this website's case, the domain is "imlearningjoomla". (Also note that many times, the domain name extension is included when we talk of domains or domain names. ie. imlearningjoomla and imlearningjoomla.com are sometimes both referred to as the domain name.)
It gets even more confusing when we start talking about sub-domains. So what is a sub-domain?
When a site gets really big, it might like to organise itself into sub domains. An couple of examples of sites that do this is about.com and wikipedia.org. All content on about.com is categorized into sub-domains. An example of a sub-domain on about.com is the "personalweb" in personalweb.about.com. On wikipedia.org, they use sub-domains for the different language version of their site. Going back to the metaphor, sub-domains would be like suburbs of a large city.
Now to get even more confusing, but just to let you know for the sake of knowing - the "www" in many website addresses is technically a sub-domain and leads people to the question of choosing to use www or non-www for their website.
If you choose to start a blog on wordpress.com and go the free route, you'll have to choose a blog name which will become your web address and will be a sub-domain or wordpress.com. Your blog name might be Homebrew Curator, in which case the name will be the subdomain to wordpress.com, with your full address being homebrewcurator.wordpress.com.
In this case, the name of your site is not the domain name - it is rather, a sub-domain. You are sharing the domain name with other sites.
Registered Domain vs Free Sub Domain
If you are starting a personal website, a sub-domain should, initially, be fine. However, if you are starting a website as another means to promote your business, you should start with an address that you aren't sharing with others. Also, you should have full control over your account. (Wordpress will put their own ads on your site if you don't pay to enjoy the ad free experience.
Sub domain addresses also look less professional and because they are longer and the domain name part of the address often has nothing to do with the name of your website and business, they are harder for visitors to remember. To find a domain name, I recommend NameCheap.com.