What is C Programming ? Why it is so popular even when it is so old ?

C is a general-purpose, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. It has since spread to many other platforms, and is now one of the most widely used programming languages. C has also greatly influenced many other popular languages, especially C++, which was originally designed as an enhancement to C. It is the most commonly used programming language for writing system software, though it is also widely used for writing applications.
The initial development of C occurred at AT&T Bell Labs between 1969 and 1973; according to Ritchie, the most creative period occurred in 1972. It was named "C" because many of its features were derived from an earlier language called "B," which according to Ken Thompson was a stripped down version of the BCPL programming language.
C's primary use is for "system programming", including implementing operating systems and embedded system applications, due to a combination of desirable characteristics such as code portability and efficiency, ability to access specific hardware addresses, ability to "pun" types to match externally imposed data access requirements, and low runtime demand on system resources.
C has also been widely used to implement end-user applications, although as applications became larger much of that development shifted to other, higher-level languages.
One consequence of C's wide acceptance and efficiency is that the compilers, libraries, and interpreters of other higher-level languages are often implemented in C.
Unfortunately, C was designed as a programming language, not as a compiler target language, and is thus less than ideal for use as an intermediate language.
There is an excellent article written on Criticism of the C programming language