I’m sure there are competing products that do the same thing, but the thing it does is pretty nifty.
I would call it a mirroring or replication tool for files on multiple computers.
You have to do two things:
1) Set up an account at getdropbox.com
2) Load the dropbox application on one or more computers.
When you load the application, it makes a special folder for you. Anything you put in that folder gets copied up to the getdropbox.com cloud. Anytime you change anything in that folder, it updates your space the cloud to match.
Also, any time anything in your space in the cloud gets updated, the application replicates the changes in the special folder on your computer.
Do this with several computers and you always have a current copy of your work no matter what computer you use. It is fairly “invisible” It just works.
Also, you can grant permission to other dropbox users on a folder-by-folder basis. So, you can share selected things with selected people in near-real time.
My friends and I used it to design a robot. Every time one of us made a change to a file, we all saw the updates within a minute or so. This was coordinated across three people and each of us had two or three computers.
I also use it to pass documents to my accountant. I keep my books for my contracting work myself, which are basically a Quickbooks file. I just copy my Quickbooks file to our shared dropbox folder and he can see my entire financial picture.