You can choose to go for a domain name that people easily can remember and associate with the business you are doing (like microsoft.com) or you can choose a domain name that consists of your main keywords for example usedcomputers.com or onlinedating.com.
Some people think that it is better to have a domain that people would directly type in the address bar. For some domain names this is correct, like brand domain names (e.g. bmw.com, apple.com, adidas.com, etc.). But ask yourself how many times people will ‘guess’ your domain name when typing it in the address bar?
If you are in a certain business with a relative high competition (in my example ‘used computers’) then always go for the keywords domain name.
When people use a search engine to find a business like you, they will type keywords in the search bar. If your domain name consists one or more keywords (or even an exact match), then your site will have an advantage above sites that doesn’t have exact keyword domain names. In fact Google places a relevancy bonus on your site. (*).
E.g. if you google for “used computers” then the first (organic) result is “www.usedcomputer.com” and somewhere else you see www.usedcomputershop.com, see what I mean?
Of course there are other factors that will determine your ranking, but exact domain words will give your site extra boost.
As an example I created not long ago a site about so-called SEO nightmares. Yes, they do exists :-).
I registered this site with the name: seonightmares.com It’s not a big site and it is not very well known. But if you type “SEO nightmares” in Google or Yahoo, I probably will be on the top. This is an example why exact keyword domain names are so powerful.
If you want to learn more details about choosing the right domain name, then I suggest you strongly to become a member of the SEOBook Training Centre. Among the ever growing list of topics, you can read very good advice about choosing the right domain name.