Dropbox is an internet based storage and filing system for your documents.
It is very handy and easy to use but in case the person you are sending files to does not have a free account that they offer you can still use to share documents. While in your account right click a file or folder you want to share and select DROPBOX > SHARE LINK. A new webpage will come up that will ask you to enter recipients email. But if you scroll down a bit more you can see a GET LINK icon. Highlight and copy the link and use in your normal email method. (yahoo, google, outlook)
Unless you want people to change your files you don't need them to have a dropbox account.