C99 is a standard for the C programming language, officially released in 1999. It introduced several new features and improvements over its predecessor, C90, including support for inline functions, variable-length arrays, and new data types like long long int. These enhancements aimed to make C more powerful and flexible for developers. The C99 standard also improved the language's support for complex numbers and added new library functions. It emphasized better code portability and encouraged the use of modern programming practices, making it easier for programmers to write efficient and maintainable code in the C language.