In C programming, a macro is a fragment of code that can be defined and then used in multiple places within a program. Macros are typically defined using the #define preprocessor directive, which allows the programmer to define a symbolic name for a specific value or block of code.

Macros are expanded by the C preprocessor, which is a separate tool that runs before the compiler. During the preprocessing stage, the preprocessor replaces all occurrences of the macro with the corresponding expansion, which can be a value, a block of code, or a combination of both.

There are two main types of macros in C programming:

1. Object-like macros: These are defined using the #define directive, followed by the name of the macro and the replacement text, which can be a value or a block of code. For example:

#define PI 3.14159
#define MAX(a,b) ((a) > (b) ? (a) : (b))

In the above example, PI is a simple macro that is replaced with the value 3.14159 whenever it appears in the code. MAX is a more complex macro that takes two arguments and returns the larger of the two.

2. Function-like macros: These are similar to object-like macros, but they take arguments and can have multiple replacement texts, depending on the values of the arguments. Function-like macros are defined using the #define directive, followed by the name of the macro, the argument list in parentheses, and the replacement text, which can include references to the arguments. For example:

#define SQUARE(x) ((x) * (x))

In the above example, SQUARE is a function-like macro that takes one argument and returns the square of that argument.

Macros are a powerful tool in C programming, but they can also be misused and lead to hard-to-debug code. It's important to use macros judiciously and to follow best practices, such as using all-caps names for macros and avoiding complex or nested macro definitions.