Form builder is a Dreamweaver tool you can use to create as many forms as you want to do whatever you want.
Registration Solution is a completed site created using our tools that allows user registration and login and includes the contact solution.
The Contact Solution is a single page form that sends an email. It is similar to what you could produce using form Builder and Universal Email.
The difference is like the difference between buying a home and buying all of the materials and machinary used to build one. If you a happy with a pre-fabricated model then it is good to go with a solution. If you want to do a lot of customization you would want to have the tools used to create it.