Programming Languages – Difference Between a Statement and a Keyword

Keywords have a special meaning in a language, and are part of the syntax.

Reserved words are words that cannot be used as identifiers (variables, functions, etc.), because they are reserved by the language.

In practice most keywords are reserved words and vice versa. But because they're two different things it may happen that a keyword is not a reserved word (e.g. a keyword only has meaning in a special context, and can therefore be used as an identifier), or a reserved word is not a keyword (e.g. because it is reserved for future use).

Update: Some examples as given by others that illustrate the distinction:

• In Java, goto is a reserved word but not a keyword (as a consequence, you cannot use it at all)
• Fortran has no reserved words, all keywords (if, then, etc.) can be used as identifiers

It's not necessary -- Fortran didn't reserve any words, so things like:

if if .eq. then then if = else else then = if endif

are complete legal. This not only makes the language hard for the compiler to parse, but often almost impossible for a person to read or spot errors. for example, consider classic Fortran (say, up through Fortran 77 -- I haven't used it recently, but at least hope they've fixed a few things like this in more recent standards). A Fortran DO loop looks like this:

DO 10 I = 1,10

Without them being side-by-side, you can probably see how you'd miss how this was different:

DO 10 I = 1.10

Unfortunately, the latter isn't a DO loop at all -- it's a simple assignment of the value 1.10 to a variable named DO 10 I (yes, it also allows spaces in a name). Since Fortran also supports implicit (undeclared) variables, this is (or was) all perfectly legal, and some compilers would even accept it without a warning!