DoS attack (for Denial of Service) and DDoS attack (for Distributed Denial of Service) are the types of computer systems attacks. Their goal is to prevent or complicate legit users to access the resources provided by the system.
DoS attacks can be performed by a hacker using one computer. DDoS attack involves many computers. Typically, they are computers of ordinary trustful users. Their computers run the software that allows to control computers remotely - by a hacker.