The buying and selling of domain names has become an industry unto itself. They can't be owned. They can only be leased. You don't pay your lease and you lose your domain name. In fact, there are so many names being created, more than 25,000 domain names expire every day! And since domain names are used for marketing purposes (remember Paulspainting.com), there is a brisk market for reselling the names as soon as they expire. This means that tens of millions of dollars change hands each year with the resale of domain names.
As of 2008, the four major registrars have all sub-contracted their expiring domain lists to certain reseller and auctioneer partnerships, for the purpose of keeping the domain name at the original registrar and continuing to extract revenue off the renewal of premium registered names.
People buy the names, throw up a search engine directory on them, and offer them for resale at a much higher price that they paid for them. You have probably clicked on a link that took you to a page of links based on the subject you were searching for. That is a reseller's site. Because this practice has become so widespread, if you forget to renew your domain name, you will probably have to pay a pretty penny to get it back.
There is another group of people that register domain names that resemble a trademark in order to profit from visitors looking for that address. They will register domain names that sound like a major company, but is spelled phonetically different. For instance, they might register pepse.com in the hopes that you might type pepse in the search box, even though they are really looking for pepsi. Every time someone clicks a link on one of those search link pages, the owner gets paid a commission. Considering the number of spelling mistakes made each day, this translates in to big money.
This practice of registering names resembling trademarks has become known as cybersquatting and various laws were enacted to allow abusive registrations to be forcibly transferred, but these were sometimes themselves abused by overzealous companies committing reverse domain hijacking against domain users who had legitimate grounds to hold their names. Such legitimate uses could include the use of generic words that are contained within a trademark, but used in a particular context within the trademark, or their use in the context of fan or protest sites with free speech right of their own. Makes your head spin, doesn't it?
For those who want a particular domain name that has already been registered, many of the major registrars allow you to add your name to a list to buy that name if it expires. It will cost you more than a new one, but if it has marketing value to your business, it may be something to consider.