Please help I've several questions : what are precompiled Headers ? what is their usage ? how to make one ? and how to include one ?
Precompiled headers (PCH for short) are something that the some compilers support. The support and what they contain [aside from "something that hopefully can be read quicker than the original header file"] is up to each compiler producer to decide. I have a little bit of understanding of how Clang does it's precompiled headers, and it's basically a binary form of the "parsed" C or C++ code in the header - so it produces a single file that doesn't need parsing [to the same level as the header file itself].
The purpose is to reduce the compile-time. However, in my experience, the LONG part of the compilation is typically code-generation with optimisation. However, in some instances, especially when LOTS of header-files are involved, the time to read and parse the header files can be a noticeable part of the overall compilation time.
Generally speaking, how they are used is that you tell the compiler that you want precompiled header, and for each compilation, the compiler will generate the precompiled header if it's not already there, and read it in when it is present [1] - commonly this is done for one named header file, which includes lots of other things. Microsoft Visual Studio typically has a file called "stdafx.h" that is precompiled - and at least in the case of MS products, this has to be the first file that is inclduded in a project [this is so that no other header file for example changes the meaning of some macro - I expect there is a hash of the compiler/command-line definitions of macros, so if one of those changes, the PCH is recompiled].
The idea is not to include every single header-file in this one precompiled file, but header files that are used in MOST files, and that are not changing often (the PCH needs to be regenerated if one if the files that is precompiled has changed, so it's no point in doing that if you keep changing the header-files frequently). Of course, like any other build dependency, anything using the precompiled header will need to be rebuilt if the PCH has changed.
For exactly how to use this, you will need to read the documentation for the compiler you are using.
[1] If nothing has changed that requires it to be rebuilt.