Network Solutions, GoDaddy, Register.com and Domain.com are just a few of the hundreds of companies that you can now buy your domain name from. They are called "registrars."
Each of them are trying to improve their profits, yet the prices of domain names have dropped as low as $1.99 for the .info domains. As a sales tactic, each of these registrars have started promotions to have you buy more and more domain names.
"Secure your biz, co, net, org domain name and protect your trademark" is a common slogan. But how useful is this really? Why would you want 5 domains, or 15 domains?
In reality, you only ever want to advertise one primary domain name. If you have jewelrystores.com and jewelrystore.net you should simply use the .com version. The .net becomes useful only for special purpose advertising where you can track that extra domain name, like a post card mailing or newspaper ad.
On the other hand, if you are interested in a more aggressive marketing strategy, you could register different domain names and create domain specific websites to match.
For example, you could register BestMensRings.com and build a website dedicated to selling various types of men's stainless steel rings.
Here are some other ideas to help you understand the idea.
The .mobi domain name is more and more popular, and it represents mobile websites. Savvy smartphone users understand that a .mobi website will work on their cell phone.
If you do register a .mobi domain, you should not simply direct it to your normal website. Doing so is not only false advertising, but also against the international domain registration committee's regulations.
The bottom line today is that it's your option to protect your trademark by registering multiple versions of your domain names. If you register special phrase domains, you should create special websites to match; and if you register a .mobi you should use it only on a mobile site.