What is the difference between Debug and Release in Visual Studio?

The most important thing is that in Debug mode there are no optimizations, while in Release mode there are optimizations. This is important because the compiler is very advanced and can do some pretty tricky low-level improving of your code. As a result some lines of your code might get left without any instructions at all, or some might get all mixed up. Step-by-step debugging would be impossible. Also, local variables are often optimized in mysterious ways, so Watches and QuickWatches often don't work because the variable is "optimized away". And there are multitudes of other optimizations too. Try debugging optimized .NET code sometime and you'll see.

Another key difference is that because of this the default Release settings don't bother with generating extensive debug symbol information. That's the .PDB file you might have noticed and it allows the debugger to figure out which assembly instructions corresspond to which line of code, etc.

If you go through project compile options and compare them, you'd see what are the differences.

Assuming the question is about native/C++ code (it's not entirely clear from the phrasing):

Basically, in Debug all code generation optimizations are off. Some libraries (e.g. STL) default to stricter error checking (e.g. debug iterators). More debugging information is generated (e.g. for "Edit and Continue"). More things are generated in code to catch errors (local variable values are set to an uninitialized pattern, and the debug heap is used).

"Debug" and "Release" are actually just two labels for a whole slew of settings that can affect your build and debugging.

In "Debug" mode you usually have the following:

• Program Debug Database files, which allow you to follow the execution of the program quite closely in the source during run-time.
• All optimizations turned off, which allows you to inspect the value of variables and trace into functions that might otherwise have been optimized away or in-lined
• A _DEBUG preprocessor definition that allows you to write code that acts differently in debug mode compared to release, for example to instrument ASSERTs that should only be used while debugging
• Linking to libraries that have also been compiled with debugging options on, which are usually not deployed to actual customers (for reasons of size and security)

In "Release" mode optimizations are turned on (though there are multiple options available) and the _DEBUG preprocessor definition is not defined. Usually you will still want to generate the PDB files though, because it's highly useful to be able to "debug" in release mode when things are running faster.

Mostly, debug includes a lot of extra information useful when debugging. In release mode, this is all cut and traded for performance.