All computers' hosts can only be differentiated by IP address, with the explosion of the number of Internet hosts. It is obviously hard for users to remember these IP addresses; therefore, domain name, a unique name that identifies an Internet site, plays the role of conversion mechanism.
No, in general, domain name is registered in First Come First Serve basis; you cannot registrar a domain name which is already registered by others.
Domain name is your online identification. Registering and using more than one domain name will help you to protect your brand name and build your business on internet. You can registrar a domain name not only with your brand name but also with keyword of your business.