DDOS

To start with this topic I must first explain the concept of DOS.

DOS stands for denial of service, this is done by having a computer connect to a website repetatively to use up the hosting servers resources. It does this since connecting with that server is part of a 3 way 'handshake'. First the device must request connection to the server, the server must then send a signal to the device telling it that it has opened availability to connection, then the device connects to the server. With a DOS attack, the device will only complete the first 2 stages of this handshake. This leaves the server staying open for the device to connect to it. If this happens from the same device many times, the server will then use all of its resources trying to keep the connections open for this device.
Nowadays the server would have protocols which would allow it to drop these connections after a set amount of time, thus freeing up resources. This leads on to DDOS attacks.

DDOS stands for distributed denial of service. This involves many devices performing a DOS attack on the same target. Since modern servers have the protocols which allow them to drop the connection after a set amlunt of time, the DDOS must have enough devices for the attack to use up the servers resources within that time limit.
Therefore hackers could employ a botnet in order to help them with their DDOS efforts.