An attempt to take a website or user off the internets by way of brute force alone (i.e no computer science III). It more often than not is simply flooding the target system with ping requests, but it is also possible to use other types of packets destined for any service on the target system that is accepting connections. To make it even better, the iconic DoS attack comes in several /b/tastic flavours. In order to understand how Denial-of-Service attacks are carried out, it is necessary to understand a bit about how connections are created over the internet.
Please note that people ddosing other players on Minecraft doesn't mean shit. A real ddos attack means taking down a companies website and costing them thousands of dollars in damage.