DoS = Denial Of Service
DDoS = Distributed Denial Of Service
What is the difference between the two?
Whilst DoS and DDoS sound remarkably similar there are in fact differences between the two -
A DoS Attack is a Denial of Service attack.
This means that one computer and one internet connection is used to flood a server with packets (TCP / UDP). The point of such a denial of service attack is to overload the targeted server’s bandwidth and other resources. This will make the server inaccessible to others, thereby blocking the website or whatever else is hosted there.
A DDoS Attack is a Distributed Denial of Service Attack.
In most respects it is similar to a DoS attack but the results are much, much different. Instead of one computer and one internet connection the DDoS attack utilises many computers and many connections. The computers behind such an attack are often distributed around the whole world and will be part of what is known as a botnet. The main difference between a DDoS attack vs a DoS attack, therefore, is that the target server will be overload by hundreds or even thousands of requests in the case of the former as opposed to just one attacker in the case of the latter.
Therefore it is much, much harder for a server to withstand a DDoS attack as opposed to the simpler DoS incursion.