what's the difference between mt-gd and mt-s library

Read Boost Getting Started on Windows

• mt : multi threaded
• d : Add ABI tags, could be used with:
  • g : using debug versions of the standard and runtime support libraries.
  • s : linking statically to the standard and runtime support libraries.
  • and more

As pointed out in the other answers and comments, these represent different configurations. With this answer, I'd like to give a more complete overview and link to the corresponding Visual Studio configurations:

1. Boost's -mt-s corresponds to VS' Runtime Library setting /MT
2. Boost's -mt-sgd corresponds to VS' Runtime Library setting /MTd
3. Boost's -mt corresponds to VS' Runtime Library setting /MD
4. Boost's -mt-gd corresponds to VS' Runtime Library setting /MDd

First and second can be built with ./b2 runtime-link=static threading=multi
Third and fourth can be built with ./b2 runtime-link=shared threading=multi