A domain name becomes your Brand! If wisely chosen, it can help you in branding, traffic generation & also online revenue generation, which is simply the outcome of the earlier two, you already have in your mind. While choosing a domain name one should take care of some points, which I have observed are beneficial for businesses.
How Many Domain Names should be owned for a single websiteA single domain name is sufficient in most of the cases. None of your serious competitor would like to buy anything similar to yours, but as you grow, someone may start notoriety near your domain. So, you may like to block this possibility. Such notorious people seldom grow beyond a limit, but they do cause casual harrassment. Cybersquatters & Phishing criminals are only looking for such an opportunity.
Keywords in domain nameDomain with an industry specific keyword, certainly helps in search engine rankings. This effect is waning with every SERP algorithmic changes, but some advantages are inherent which can't be nullified like CTR gain due to bolding and instant trust gain due to sheer effect of name. Grab it if you get the chance.
Domain Name Registrar choiceThere are vast number of registrars nowadays dealing in domain names. Choose a good service provider with good interface. A cheap one with horrible interface are nightmare when it comes to configuring them. Sometimes, you shall have to shift away from them when you find them incompatible for some services. Stick with reputed ones.
Redirection of DomainsA permanent redirection of one domain to another may give you backlink and PR boost. Choose some old one from your own industry, to get good benefit. Like ITmesh.com redirects to netargument.com
Branding, email, blogging... domains for the firmThe domain which hosts your website is your branding domain. It is your face of business. It might also be abused by notorious people who may spoof your identity. So, to emphasize credibility and for security reasons, some companies prefer a different domain for email, like .net if .com is widely known. Some subdomains like corp.xyz.com, mail.xyz.com etc. It is fashionable to blog on xyz.com/blog for a corporate blog.
Domain name vis a vis BrandingDomain should be markatable. While choosing a domain name one should always have branding in his mind. Preferably it should be a .com extension of your business name or better first find a domain name before branding your business.