A Distributed Denial of Service (DDoS) attack is something that's supposed to be sustained and impactful. However, in regards to how easy it is to conduct a DDoS, here are some things you need to keep in mind to realize the effort (or lack thereof) it takes to make it possible. Many people can figure out how to conduct a DDoS in 10 minutes or less. Wikipedia can even guide you through the process, along with many other hacker sites that intentionally teach the easy, script-kiddie-friendly way to do it.
Simple concept behind DDoS and DoS
The reason why DoS and DDoS attacks are effortless to execute is because the idea behind the concept is simple. Find a way to send thousands of tiny packets at a person's computer in order to overload it and make it inaccessible. It's all about hogging resources and making a machine inoperable because of wave upon wave of requests that by themselves aren't hard to process, but when used collectively can cripple entire servers and websites. With that said, a DoS attack only comes from one location, which means with a simple IP block, it can be thwarted. This goes double for script kiddies who don't really know what they're doing.
Multiple Location DoS Attack
A DoS attack may end up barely making a dent on a computer if the one executing it doesn't know what he's doing or he only learned about how to conduct a DoS attack just now from Google and is going about it wrong. Meanwhile, a DDoS attack is much harder to deal with because it involves the cooperation of multiple users or even botnet-infected machines that come from multiple locations. It doesn't matter how; as long as the DoS attack is coming from multiple locations, then it's considered a DDoS attack.
In that regard, DoS isn't as easy to execute as it is to learn, and in order to be devastating, it's best done with help from multiple machines in different locations. By having DDoS attacks come from all corners, so to speak, it will be harder for a web administrator to block the wave upon wave of packets that's eating up resources and making the webpage unavailable for people to see. There are routers capable of blocking the subnet range. However, if there are hundreds of subnets (or botnets) involved, you're likelier to block the IP of a website, application, or person that was merely assigned to the subnet without solving your DDoS attack problem in the least.