The domain you use for a your web site can have a huge impact in the way that humans and search engine spiders perceive it. Domain names were once so expensive that only those wanting to protect a brand or who could afford them owned them, now they are commonplace and if you know where to look, you could pick up a domain name for free. Now that they are much easier to obtain, it is hard to take any site that does not have it's own domain name very seriously. Without a domain name, a company is likely to be passed off as amateurish or seen to want to cut corners to save money. If you have a company site and you still haven't got a domain name for your website, then you might want to think about getting one!
The best domain names are those that people can remember, because if your visitors can remember it, they are more likely to come back. If you do not already have a company or other name selected for your site, then it is often a good idea to make your domain name the brand for the site. Tying your site name to the domain is just one way you can help your visitors remember how to find you again.
One of the major problems of trying to promote a specific domain name is that in general, people (or sometimes browsers) assume that they end in a.com. If your site uses a domain other than a.com, then you may want to make the domain your site identity, otherwise you could have your visitors remembering the domain name but not the extension and going to related site that isn't your own!
If you purchase your domain with the intention that it will become the brand that you promote, then it is worth taking into account the specifics of the domain. Keywords can play an important part in your brand, particularly if it is specific to your industry, as they will help to make your domain name memorable while improving your search engine rankings.
Another thing to avoid is the use of "cool" domains; using numbers or letters to replace words is only going to confuse anyone who wants to find your site by typing in your domain. If you really must register such a domain, then be sure to register the expanded form and point it to the same site so as to pick up any visitors that were unsure which to use. Competition for domain names is higher than ever since the average price of a domain fell, and so there is a chance that your ideal domain name has been taken. The longer you wait before buying your domain name registration, the more difficult it will be to make the changes later on down the line. Fortunately most paid web hosting companies will require you to either use a registered domain name with the account or to register one through them; which will help to minimize these sorts of problems in the future.