The answer to this is platform-specific; what happens on Linux is different from what happens on Solaris, for example.
The easy part (because it is not platform-specific) is the separation of 'gcc' and 'g++':
- gcc is the GNU C Compiler from the GCC (GNU Compiler Collection).
- g++ is the GNU C++ Compiler from the GCC.
The hard part, because it is platform-specific, is the meaning of 'CC' (and 'cc').
- On Solaris, CC is normally the name of the Sun C++ compiler.
- On Solaris, cc is normally the name of the Sun C compiler.
- On Linux, if it exists, CC is probably a link to g++.
- On Linux, cc is a link to gcc.
However, even on Solaris, it could be that cc is the old BSD-based C compiler from /usr/ucb. In practice, that usually isn't installed and there's just a stub that fails, wreaking havoc on those who try to compile and install self-configuring software.
On HP-UX, the default 'cc' is still a K&R-only C compiler installed to permit relinking of the kernel when necessary, and unusable for modern software work because it doesn't support standard C. You have to use alternative compiler names ('acc' IIRC). Similarly, on AIX, the system C compiler goes by names such as 'xlc' or 'xlc32'.
Classically, the default system compiler was called 'cc' and self-configuring software falls back on that name when it doesn't know what else to use.
POSIX attempted to legislate its way around this by requiring the programs c89 (originally) and later c99 to exist; these are the compilers compatible with the ISO/IEC 9899:1989 and 9899:1999 C standards. It is doubtful that POSIX succeeded.
The question asks about the differences in terms of features and libraries. As before, the answer is platform specific in part, and generic in part.
The big divide is between the C compilers and the C++ compilers. The C++ compilers will accept C++ programs and will not compile arbitrary C programs. (Although it is possible to write C in a subset that is also understood by C++, many C programs are not valid C++ programs). Similarly, the C compilers will accept C programs and will reject most C++ programs (because most C++ programs use constructs not available in C).
The set of libraries available for use depends on the language. C++ programs can usually use C libraries on a given platform; C programs cannot usually use C++ libraries. So, C++ has a larger set of libraries available.
Note that if you are on Solaris, the object code produced by CC is not compatible with the object code produced by g++ -- they are two separate compilers with separate conventions for things such as exception handling and name mangling (and the name mangling is deliberately different to ensure that incompatible object files are not linked together!). This means that if you want to use a library compiled with CC, you must compile your whole program with CC. It also means that if you want to use one library compiled with CC and another compiled with g++, you are out of luck. You have to recompile one of the libraries at least.
In terms of quality of assembler generated, the GCC (GNU Compiler Collection) does a very good job. But sometimes the native compilers work a bit better. The Intel compilers have more extensive optimizations that have not yet been replicated in GCC, I believe. But any such pontifications are hazardous while we do not know what platform you are concerned with.
In terms of language features, the compilers all generally hew fairly close to the current standards (C++98, C++2003, C99), but there are usually small differences between the standard language and the language supported by the compiler. The older C89 standard support is essentially the same (and complete) for all C compilers. There are differences in the darker corners of the language. You need to understand 'undefined behaviour', 'system defined behaviour' and 'unspecified behaviour'; if you invoke undefined behaviour, you will get different results at different times. There are also many options (especially with the GCC) to tweak the behaviour of the compiler. The GCC has a variety of extensions that make life simpler if you know you are only targetting that compiler family.
