A DDoS attack (denial-of-service attack) means numerous illegal computers access an Internet server at the same time and the huge incoming traffic flooding makes the server unable to response legal users. It is like thousands of people rush in and occupy a Walmart store without buying anything. From the technical perspective, DDoS attacks can work on each layer of Network protocols. Please find more on Denial-of-service attack.
Why servers get DDoS attacks? There are many reasons. Business competitors might launch DDoS attacks to beat you death and bring traffic to them. Anyway, the Internet nowadays is in its wild early age and it is much easier to do evil than to do good on the Internet. Angry customers or revenge might also lead to DDoS attacks. Some guys do it for fun, because this game is cheap and no penalty.
Does DDoS attacks cost you much? Absolutely. According to a study, the cost is up to $50,000 per attack. Large enterprises need to invest much on DDoS protection systems and they should have a new department - cyber security team. That is big price.