DropBox is an online storage/sharing/syncing site for files, and is useful for a great many file related tasks.
CloudOn and OnLive are virtual desktop computers. You use your iPad to access them and run programs remotely (Mostly the MS Office suite). The programs available are slightly different on each of the services. Both of these services have DropBox integration, which is how you move files between your iPad and the virtual desktop. One of them, don't remember which, also supports Box.com, a service similar to DropBox but with more emphasis on business features.