Amit Said..
Preprocessor is a program that processes its input data to produce output that is used as input to another program. The output is said to be a preprocessed form of the input data, which is often used by some subsequent programs like compilers. Preprocessor directives are instructions to the compiler and its lines begin with a pound (#). They are executed before the compilation of the code begins. These directives can only span a single line and does not need to end in a semicolon.
The preprocessor contains directives like #define, #include, #error etc
#if ,#elif, #endif, #else directives are conditional directives