Of all the attacks that can be coordinated against a target, one of the easiest to attempt successfully is a Denial of Service (DoS) attack. This has a lot to do with the brute force nature of these attacks.
At the simplest level, DoS attacks require a device with network connectivity to send a large amount of traffic towards another network connected device. At their most complex, these types of attacks become Distributed DoS (DDoS) attacks with multiple attacking devices coordinated via a distributed control structure.
