What are Diagnostic Tools?
First let’s define what we mean when we say “Diagnostic Tools” in Visual Studio. Diagnostic tools means different things to different people, and some would consider the debugger itself to be a diagnostic tool. In this context, when we say Diagnostic Tools, we mean tools that allow you to see historical information (data collected over time), as opposed to just a single moment in time (like when you are stopped at a live breakpoint). By that definition, there are three main examples of diagnostic tools that were available previously in Visual Studio 2013:
The Output window: allows you to see program output and other debugger events while it is running
IntelliTrace: allows you to see interesting events in your application and set the debugger back in time so that you can look at historical data in the Call Stack, and Locals windows. You can also see the list of every method call and its parameters (even when that method is no longer on any call stack).
The Performance and Diagnostics hub: allows you to perform a comprehensive performance analysis of your app as it runs without the debugger. You have your choice of performance tools to use, such as the CPU Usage tool and the Memory Usage tool, depending on the nature of the performance issue.
As you will see, we have taken all of these diagnostic tools and brought them closer together into a unified window. This new experience offers you two benefits:
The ability to monitor performance while debugging, and correlate performance data with debugging activity, and
A much richer and more useful experience for IntelliTrace and the Output window, dramatically shortening the time it takes you to diagnose an issue or verify a fix.
Elliot Balynn's Blog 