Your domain is like your address on the internet. Just like you can have a building with one address but multiple appartments or offices, your domain can have an infinite number of pages. The important thing about a domain is making it memorable to other people.

You want your domain choice to fit with the content of your site. If you want to make a site for to compare hosting plans you could choose the domain graydaysinmay.com, but that wouldn't have anything to do with your content. This is widely viewed as bad form on the internet. You want to choose a domain like the one above www.hostsearchsit.com. It doesn't contain a full sentence but it kind of flows and contains key words that someone can remember when they think of the content on the site.

A domain needs to point to a computer somewhere in the world that hosts (or stores files to feed the website visitor) the website.  If you plan on maintaining a large number of websites, you should get a web hosting plan that allows for more domains than you plan on using.  Here is a good list of unlimited domain hosting options.

If you don't have a domain yet and you just want to create one site - your best bet is to get a web hosting plan that includes a domain in the cost of the hosting plan.