I will try to illustrate it with an example since it all will depend on the type of application you would like to implement. Let's say you would like to implement an application where you log data, maybe exchange data with databases and you acquire a bunch of data points at a time not performing any type of control and where high determinism is not needed. In this case LabVIEW for Windows work great. If you on the other hand would like to implement a closed-loop application where you need high determinism and reliability and will acquire data point by point you would use LabVIEW RT which is an add-on to LabVIEW. I.e. you need LabVIEW + LabVIEW RT when developing an RT application.