Distributed Denial-of-Service attack, also known as DDos attack performed to take down a server. In DDoS attack, a web server flooded with malicious web requests or traffic. Multiple compromised systems are used by attackers or hackers for DDoS attack. DDoS attack can disrupt a service for several time.
To understand DDoS attack better, take a example of road traffic jam. In the same way, when a server receives http requests larger than its capacity then it fails to handle requests.