NDA only gives you protection from the individual in the contract, in which they promise to keep the idea confidential. A patent grants you proof of ownership and full protection for your idea. The main difference can be shown by an example – let’s say you have an idea to develop something and you share that idea with person A and sign an NDA with them. If person A goes and makes your invention “as their own”, you can sue them and win. However, if person A goes and tells person B about the idea and they make the invention, you can’t do anything about that. Thus patent is a higher level of protection and ownership of your idea, and grants you protection from anyone who may come up with a similar idea.