A DDOS is a large-scale, frequent series of DOS attacks to prevent people from accessing a network. A DOS is a denial of service attack, it's more of a phenomenon with many strategies. DOS attacks target DNS servers, which converts a URL (i.e. this forum page) to an IP address (since humans are better at memorizing words than numbers) for computers and servers to communicate. When the DNS server is attacked in some way and unable to respond, people can no longer connect to the network. A DDOS is worse in that it targets root servers names, which could prevent access to numerous, unrelated websites. Servers have very powerful security measures in place to prevent these attacks as best as possible and ISPs (Internet service providers) have additional measures.

There's much more complexity to this, including root access (computer speak for gaining administrative power over the network).