I am an AVID user of DropBox. You can do some Google searches on it and find ways that people are using it. I currently have a free account which actually gives you a fair amount of space, but you can get a lot more space for a monthly fee. I absolutely love it.
What is great about it is that you can access your files from anywhere, any computer with internet access. If you have more than one computer or device, it will sync the files across all of them. For example.....at home there's my PC, DH's PC, our laptop and my iPhone. The computers all have DropBox installed on them and I can access and modify the files on any of them and move between. I have an iPhone app which allows me view-only capabilities.
It's file sharing but you also have a back up because the files are continuously backed up.
If there's a folder you want to share with someone, you can do that (photos or a document that you both need to make changes to, for instance).
Also comes in handy for college courses I took this fall. Had a presentation to give w/ a partner. She downloaded the DropBox application to her PC, I shared our presentation folder with her, and then we were both able to make changes to it.
The DropBox folder on your computers "live" under My Documents. So you save files to that folder - even though it looks like it's physically in your computer's hard drive, it's really not! I hope that makes sense.
