The GCC package contains compilers, preprocessors and the GNU C++ Library.
A compiler translates source code in text format to a format that a computer understands. After a source code file is compiled into an object file, a linker will create an executable file from one or more of these compiler generated object files.
A preprocessor pre-processes a source file, such as including the contents of header files into the source file. It's a good idea to not do this manually to save a lot of time. Someone just inserts a line like #include <filename>. The preprocessor inserts the contents of that file into the source file. That's one of the things a preprocessor does.
The C++ library is used by C++ programs. The C++ library contains functions that are frequently used in C++ programs. This way the programmer doesn't have to write certain functions (such as writing a string of text to the screen) from scratch every time he creates a program.