How do floods work?

Answer 1

By "Floods" I take it you mean network floods.

Network floods usually involve some type of DDoS attack (Distributed Denial of Service). This is where multiple (often thousands) of computers try to connect to a single machine. All the "zombie" machines have been infected with a virus or program that allows the hacker to remotely initiate the attack.

Eventually the destination machine can nolonger support the volume of traffic, and will either bottleneck, and only accept as many clients as it can handle, or crash. Either way, legitimate users cannot access the machine!