C v/s ANSI C v/s GNU C | What's the difference?

K&R C: 

Everything before standardization is generally called "K&R C", after the famous book, with Dennis Ritchie, the inventor of the C language, as one of the authors. This was "the C language" from 1972-1989.

C89:

The first C standard was released 1989 nationally in USA, by their national standard institute ANSI. This release is called C89 or ANSI-C. From 1989-1990 this was "the C language". Please note that since 1989, ANSI haven't had anything to do with the C language. Programmers still speaking about "ANSI C" generally haven't got a clue about what it means. ISO "owns" the C language, through the standard ISO 9899.

C90:

The year after, the American standard was accepted internationally and published by ISO (ISO 9899:1990). This release is called C90. Technically, it is the same standard as C89/ANSI-C. Formally, it replaced C89/ANSI-C, making them obsolete. From 1990-1999, C90 was "the C language".

C99:

In 1999, the C standard was revised, lots of things changed (ISO 9899:1999). This version of the standard is called C99. From 1999-2011, this was "the C language".
With the demand from the developers requirements, in 1999-2000 further or additional keywords and features have been included in C99 (ex: inline, boolean.. Added floating point arthematic library functions)

C11:

In 2011, the C standard was changed again (ISO 9899:2011). This version is called C11. Various new features like _Generic, _Static_assert and thread support were added to the language. The update had a lot of focus on multi-core, multi-processing and expression sequencing. From 2011-2017, this was "the C language".

C17:

In 2017, C11 was revised and various defect reports were solved. This standard in informally called C17 and was released as ISO 9899:2018. It contains no new features, just corrections. It is the current version of the C language.

GNU C:

"GNU C" can mean two things. Either the C compiler itself that comes as part of the GNU Compiler Collection (GCC). Or it can mean the non-standard default setup that the GCC C compiler uses. If you compile with gcc program.c then you don't compile according to the C standard, but rather a non-standard GNU setup, which may be referred to as "GNU C". For example, the whole Linux kernel is made in non-standard GNU C, and not in standard C.
GNU is a unix like operating system (www.gnu.org) & somewhere GNU's project needs C programming language based on ANSI C standard. GNU use GCC (GNU Compiler Collection) compiler to compile the code. It has C library function which defines system calls such as malloc, calloc, exit...etc

This is how it looks:

AN EXAMPLE WOULD BE:

C99 valid:

for (int i = 0; i < 100; i++)
{
   doSomething();
}

C89, ANSI and older:

int i;
for (i = 0; i < 100; i++)
{
   doSomething();
}