g++ is the C++ frontend to the GNU Compiler Collection (GCC). It's the de facto standard compiler for C, C++, Go, Fortran, and Ada on Linux and supports many other languages and platforms as well. Apart from the compiler, GCC has a toolchain (libc, libstdc++, objdump, nm etc.) that is widely used on several platforms and systems.
G++ is the c++ compiler in the GNU Compiler Collection, gcc.
G++ implements most of C++98/C++03 and in recent releases also supports most of C++11. It ships with the GNU implementation of the C++ standard library, libstdc++. Like the rest of GCC, it is free software and runs on a wide range of operating systems and architectures.
See also GCC online documentation.