DDoS (Denial of Service) attacks means service denial by interrupting certain services, functionalities, and traffic through various technical methods. This type of attack prevents or impairs the authorized use by making the business service unavailable for legitimate users. Here the origin of the attack mostly is one or a few sources, and the attack size may be minimal, but the impact is significant.
However, Distributed Denial of Service took a different turn and form during last few years, where the source of an attack become extremely distributed, in a very co-ordinated manner with a common goal in the target. This kind of attack uses a huge number of source computers, which are malware infected, and is under the control of the attack originator or the force that executes the attack.
The recently executed DDoS attack of the size of 1 Tbps was with the use of malware-infected smart devices; that might have included CCTV cameras, recorders, thermostats, routers, and Cars.
In most of the cases, a DoS (Denial of Service) attack uses one computer and one Internet connection (or a few), and a Distributed Denial of Service uses multiple computers (devices) and Internet connection, that spread across the cyber world, and without any geographic limitation.