这三个版本的区别在于编译出来的目标文件是否包含调试信息(包含调试信息的多少)，其中debug版本是对每个文件都增加调试信息，不进行优化，debugrel属于中间版本，是对某些文件增加调试信息，进行部分优化，release版本是最新版也就是最后的发行版，是将所有的代码进行优化，去除所有的调试信息。
下面是英文说明：
Debug
This build target is configured to build output binaries that are fully debuggable, at the expense of optimization. Use if you plan to build separate Debug and Release<BR>versions of your code. This build target provides the best debug view while you are developing your code. It is also configured to output basic image information in an error and messages window.
DebugRel
This build target is configured to build output binaries that provide adequate optimization, and give an adequate view. Use it if you plan to build a version of your code for debug, and release the same code to reduce testing.
Release
This build target is configured to build output binaries that are fully optimized, at the expense of debug information. Use it if you plan to build separate Debug and Release versions of your code. This build target outputs optimized code suitable for release.