There are mainly three types of DDoS attacks:

Application Layer Attacks

DDoS Attack
It is also called a layer 7 DDoS attack. Its primary aim is exhaust system resources making them unavailable for legitimate users. The attacker runs multiple HTTP requests and drain the available system resources and make the server unavailable to process the requests. It is also called as HTTP flood attack.

Protocol Attacks

DDoS Syn Attack
Protocol attacks, also called state-exhaustion attacks, target the state table capacity of the server or resources like load balancers and firewalls. As an example, the SYN flood attack exploits the TCP handshake protocol by sending a large number of TCP SYN packets to initiate “Initial Connection Request” with the victim. These packets have forged source IP addresses. The victim machine sends response to every coming connection request and waits for the next packet of handshake data indefinitely. As a result all the  resources are exhausted in this process.

Volumetric Attacks

DDoS Attack
Here the attacker play on the bandwidth of the server and generates huge traffic thereby saturating the server bandwidth. An example of this type of this attack is “DNS Amplification Attack”. Here the attacker sends requests  to DNS server with  spoofed IP address (victim’s IP address), as a result the victim IP address start receiving large amount of response from the server which was meant for attacker.