What's the difference between Google Docs and Dropbox?

I am a teacher and want to use something web based so that students can turn in work, presentations, collaborate etc...and I've run across both programs and can't really tell if there is a discernable difference between the two.|||Google Docs is an online service that allows you to create word documents, spreadsheets, slideshows etc. Dropdox is a tool for syncing data between multiple computers.