So what you're saying is that ColdFusion Builder replaces ColdFusion Studio? I apologize if my question sounds trivial but I haven't been involved with ColdFusion since version 4.5, I have not taken the time to keep up on the latest releases and associated products.
ColdFusion 9 is a web application server. It gets installed on the same machine as a web server, typically, and will generate output when someone requests a URL that corresponds to a CF script in their browser. ColdFusion 9 is necessary to run CF programs.
ColdFusion Builder is a development environment for ColdFusion programmers. It allows programmers to write and debug the scripts you put on a CF server. While it's very helpful, it's not required to write CF programs. It also doesn't run CF programs - you need ColdFusion for that.