The C++ Standard Library is divided into many portions, each with its own header. The headers contain the function prototypes for the related functions that form each portion of the library. The headers also contain definitions of various class types and functions, as well as constants needed by those functions. A header “instructs” the compiler on how to interface with library and user-written components. Figure 6.6 lists some common C++ Standard Library headers, most of which are discussed later in the book. The term “macro” that’s used several times in Fig. 6.6 is discussed in detail in Appendix E, Preprocessor.