This version is frequently regarded as an unofficial standard for the most basic features of a C compiler. Since not all of the currently-used compilers were updated to support ANSI C fully (or have certain bugs that contradict ANSI), K&R C is often seen as the lowest common denominator that should be stuck to if maximal portability is desired. Thus, for example, the bootstrap? of the the GCC compiler (xgcc) is written in K&R C, since many of the platforms that don't have an ANSI C compiler do have an older K&R one.
By 2001, the above paragraph is probably a historical fact rather than the current situation. ANSI C is now supported by almost all the widely used compilers.
K&R C programs, if reasonably well written, are legal in ANSI C, and will normally have the same effect in the two dialects. This was one of aims of the ANSI C standard process, although they added more new features than is normal in programming language standardisation.
The C language evolved fairly continuously from its begining circa 1973 to the first ANSI C standard. K&R is a snapshot from this process. Features that were added between 1973 and K&R:
Features added after K&R but before the begining of the ANSI C process (IE, these are NOT in K&R):
In addition, several features were added in the ANSI C standardisation process itself. Most notably, function prototypes (borrowed from C++). Therefore, these were also NOT in K&R.