Cloud computing really is a revolutionary technology achievement. Don't let anyone convince you otherwise. At the end of the day, cloud computing as a technology levels the playing field among companies as it turns many functions of IT into a utility. No longer do you have to directly cover the charge of managing datacenter overhead.
Once upon a time the bigger the datacenter you had, the larger the applications you could develop and run. Now that even small companies with meager financial resources can use cloud computing and storage to build apps, the old paradigm is shattered. In The Big Switch, Nicolas Carr likens cloud computing to electricity. When Edison figured out how to turn electricity into a utility, a commodity that anyone could purchase, larger factories no longer had the strategic advantage of generating their own electricity (larger generation meant more widget production).
Here are some common SaaS/Cloud based solutions: