What is DRM?
Digital rights management (DRM) is a term for access control technologies that are used by hardware manufacturers, publishers, copyright holders and individuals to limit the use of digital content and devices. The term is used to describe any technology that inhibits uses of digital content that is not desired or intended by the content provider.
In terms of eBooks, DRM will discourage people from sharing your books with others who did not pay for it.
Should I choose DRM for my book?
This a personal preference. Some people are very opposed to the whole DRM idea, and for political reasons they choose to not use DRM – and some buyers will not BUY books using DRM. Others don't mind if their books are shared and passed around – it actually might result in more sales. But some believe illegal sharing will result in fewer sales. In the end, it's up to you. And it probably really doesn't matter all that much.
Do I need to copyright my book?
No... not unless you plan on spending a lot of time in court over it. Have a look at this short article that gives you the basics.