So what is a domain name? The quick answer, it is the name you choose to represent your business on the internet.
There are many "tips" on choosing a domain name, the most common is to pick something that fits your business (i.e. you sell oranges, therefore your domain name should be: joesoranges.com). Though it is true that naming your site to reflect your business can help people find you, it is not necessary. Take companies like Amazon.com, Safeway.com, Wayfair.com, and Twitter.com, though their domain name does not actually identify the product or service they offer, the sites are still quite successful. Back to our first point though, many business don't have the operating capital needed to promote their sites, so choosing a name that reflects your business is a good idea.
*Hint 1: Though many "common" names may be taken already, you can use indicators unique to your location to help you out. Take Joe's Oranges again. It is located in Minnesota so you could do joesorangesmn.com as your domain name.
*Hint 2: Don't get carried away. If your company name is "Joe's Super Large Organic Oranges," adding a .com after that may be too much to remember or too much to type, keeping visitors from going to your website.