I agreed with others about the differences between a patent and an NDA and how a patent gives you a legal proof of idea ownership. I would like to add to the idea of using NDA to protect ideas and inventions:
NDA can be helpful to protect certain ideas and concepts which are not patentable or easily protected by other intellectual property protection such as business concepts, marketing and advertising ideas, strategies etc. In these cases, signing an NDA is the best that one can do. NDA can also be helpful in providing an additional ground for filing patents in case your partner/member is infringing your patent.