Once installed, any file you drop into your Dropbox folder will synchronize and be available on any other computer you've installed Dropbox on, as well as from the web. Also, any changes you make to files in your Dropbox will sync to your other computers, instantly.
Let's say you accidentally delete that 8 page report that you were supposed to have for a meeting tomorrow. A handy Dropbox feature is the ability to undelete files or pull up any previous versions of it.
You can also share files and folders with whomever you choose, whether it be music with buddies, docs with colleagues or photos with grandma, even if they're not Dropbox users.
We obsess over speed, and it shows. Dropbox is very smart about the way it handles files. For example, if you make a change to a huge file, dropbox will only update the piece of the file that changed.
All files are transported over SSL and encrypted using AES-256 (banks use this too)