About DropBox :
Dropbox is a free service that lets you bring your photos, docs, and videos anywhere and share them easily. Never email yourself a file again!
So the meaning is you don’t have to emailing your file for your friends again if you don’t have time. just move your file to share folder of dropbox and get the link from your computer. the importany think you can access your file/data from other PC/Laptop or Synchronization. for case like this :
I want to send my file is hallo.doc
Go to Dropbox share folder :
choose the files and paste to Public folder
open right click the file and choose Dropbox->Copy Public link. the result like http://dl.dropbox.com/u/46640803/Hallo.docx. your link was can shared with friends.