C++ source files conventionally use one of the suffixes ".C", ".cc", "cpp", or ".cxx"; preprocessed C++ files use the suffix ".ii". The compiler recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that understands the C++ language—and in some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as C++ programs. g++ is a program that calls the compiler with the default language set to C++, and automatically specifies linking against the C++ library.
On many systems, the script g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Section 1.4, for explanations of options for languages related to C. See Section 1.5, for explanations of options that are meaningful only for C++ programs.