Is Dropbox Really Secure?

I do have several files and I always used my USB in keeping those important data. However, there came an incident that I misplaced my USB. Anyway, I noticed that my cousin is using Dropbox for her files. I was thinking to do the same too. But, do you think, I could take advantage of that in a very secure way? How will I make sure that my Dropbox can't be accessed by unauthorized person? Is there a way to protect sensitive data if I use it?