DoS and DDoS attacks 

A Denial-of-Service is an attack in which online services do not respond to connections from legitimate hosts due to their resources being overused by malicious requests. This attack becomes a Distributed Denial-of-Service (DDoS) attack when it comes from multiple sources and it becomes difficult to block the source with IP or Geo-location.

DDoS attacks usually take the form of the following:

• Volume-based attacks
• Application layer attacks
• Low-rate attacks