Dropbox is a great service that gives you access to back up and share files with your colleagues, family or the general public. The key benefit is the ability to retrieve a file from the cloud (web) in the absence of your actual computer itself. i.e. if you are at friend or out on the road and need access to a file you simply login and download the file in question.
If you have additional machines it automatically syncs the files that you add to Dropbox across all machines without you actually needing to transfer the files consciously.
Any this is a great service so please take a look and see if this is of benefit to yourselves.