On-Line Applications & Tools with Web 2.0

I can see why you would want to use on-line applications. It would save you a great deal of computer space. Begin able to download Excel documents into the application was great. I do have a few questions, and if given the time, I could probably find the answers: I wonder about the possibility of losing the information. How long will the on-line application provider keep my information? What is the limit to the amount of files I keep?