Explain what is DOS (Denial of service) attack? What are the common forms of DOS attack?

Denial of service.

n computing, a denial-of-service attack (DoS attack) is a cyber-attack where the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to the Internet

DOS does not cause any theft of information or security breach.

Denial of Service, is a malicious attack on network that is done by flooding the network with useless traffic. Although, DOS does not cause any theft of information or security breach, it can cost the website owner a great deal of money and time.