Denial of Service Attacks
Denial of Service Attacks are any type of attack on anything that keeps the legitimate owner or user from utilizing their own resources, generally by exploiting a security provision that allows only a certain number of failed attempts at access, but sometimes simply by overloading the resource or the access paths to it.
An example of the former type might be someone purposely making a number of incorrect attempts to open a digital door lock, causing it to go into a lockdown mode for some amount of time -- this might be to keep Good Guys from getting in to thwart a burglary being conducted inside.
An example of the latter type would be a person or organization conducting an attack on a computer service operated by someone they don't like by sending more traffic to the server than can be handled reliably by either the server itself or the communications paths leading to it. This sort of attack can be planned for by making the service distributed: putting servers to provide it in more than one place on more than one network and backbone.
But while service provision can be distributed, so can the attacks.