If you buy a domain name, then what you are actually doing is renting the use of that domain name for a given period of time that you can use to as a way of bringing people to your website. You will also own the right to renew that domain name and to sell on you rights to that domain name. You will need to pay the renewal fee at the end of the each period to retain your rights to the domain name or else you will lose any rights you have to using that domain name.
It is important to understand that owning a domain name does not allow you use nor abuse a trademark that does not belong to you. If you buy a domain name that is similar to a trademark and start using it in a way that infringes the rights of that trademark owner then you might have your domain name taken away from you.