Your files, anywhere
Any file you save to Dropbox also instantly saves to your computers, phones, and the Dropbox website.

-2GB of Dropbox for free, with subscriptions up to 100GB available.
-Your files are always available from the secure Dropbox website.
-Dropbox works with Windows, Mac, Linux, iPad, iPhone, Android and BlackBerry.
-Works even when offline. You always have your files, whether or not you have a connection.
-Dropbox transfers just the parts of a file that change (not the whole thing).
-Manually set bandwidth limits -- Dropbox won't hog your connection.

Simple sharing
Shared folders allow people to work together on the same projects and documents.

-Invite friends, family or teammates to a folder. It'll be as if you saved the folder to their computers.
-See other people's changes instantly.
-Create photo galleries viewable by anyone you choose.
-Send a link to any file in your Dropbox using your Public folder.
Dropbox mobile
Apps for iPhone, iPad, Android, and BlackBerry keep your Dropbox at hand, even on the go.

-Bring your files with you when you're on the go.
-Edit files in your Dropbox from your phone.
-Easily upload your photos and videos to Dropbox.
-Share freely with family and friends.

Your stuff is safe
Dropbox protects your files without you needing to think about it.

-Dropbox keeps a one-month history of your work.
-Any changes can be undone, and files can be undeleted.
-Secure Sockets Layer (SSL) and AES-256 bit encryption.

Sources: http://www.dropbox.com/features