File-sharing is a complicated beast. Some files are too big to send in emails; some need to be seen by a lot of people; some need to be private; some should be public. There are a ton of different applications that want to be the host for all of your files, but drop.io stands ahead of the pack.
At its simplest, drop.io is a place to store files on the Internet. You create "drops" filled with files, photos, audio, video and more that you can share with other people. There's a 100MB limit on a given drop, but you can purchase more space if you want. Typically, though, 100MB is plenty- that's an awful lot of PDFs and Word documents.