Dropbox is a program/service that lets gives you a folder on your computer that syncs to their servers. You can put the same folder on a number of different computers and they will all stay synced together. So you put a file in your Dropbox folder at work and it shows up in your Dropbox folder at home. You can also share folders with other people and it has some nice features for viewing photos.
All in all, it makes a great way to move files around without using sneaker net and is a good way to backup important data with a minimal amount of effort. The program runs on OS X, Mac and Linux so it makes it easy to move things between different systems.
One of the nice things about Dropbox is how simple it is. You can easily explain it to someone who isn’t technical and they can have it up and running in just a few minutes. It doesn’t have many options so there isn’t much to confuse people.
In addition to the files being available in your Dropbox folder, they are also available online, so you can get to important items while you are on the road away from your computer or if you need an important file while your machine is in the shop.
Dropbox also can handle different versions of a file, so you can go back to a previous version of your file if you somehow mess up the current version. This is a great lifesaver if you accidentally overwrite a file with an older version or accidentally delete the contents.
For more detailed information on Dropbox, see this Dropbox review.