What's the difference between gcc and g++/gcc-c++?

gcc

 will
compile C source files as C and C++ source files as C++ if the file has an appropriate extension; however it will not link in the C++ library automatically.

g++

 will
automatically include the C++ library; by default it will also compile files with extensions that indicate they are C source as C++, instead of as C.

From http://gcc.gnu.org/onlinedocs/gcc/Invoking-G_002b_002b.html#Invoking-G_002b_002b:

C++ source files conventionally use one of the suffixes 

.C',

.cc', 

.cpp',

.CPP', 

.c++',

.cp',
or

.cxx'; C++ header files often
use

.hh', 

.hpp',

.H',
or (for shared template code) 

.tcc';
and preprocessed C++ files use the suffix

.ii'. GCC 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, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats

.c',

.h'
and 

.i' files as C++ source files
instead of C source files unless -x is used, and automatically specifies linking against the C++ library. This program is also useful when precompiling a C header file with a

.h' extension for use in C++ compilations.

For example, to compile a simple C++ program that writes to the 

std::cout

 stream,
I can use either (MinGW on Windows):

g++ -o test.exe test.cpp

gcc -o test.exe test.cpp -lstdc++

But if I try:

gcc -o test.exe test.cpp

I get undefined references at link time.

And for the other difference, the following C program:

#include <stdlib.h>
#include <stdio.h>

int main()
{
int* new;
int* p = malloc(sizeof(int));

*p = 42;
new = p;

printf("The answer: %d\n", *new);

return 0;
}

compiles and runs fine using:

gcc -o test.exe test.c

But gives several errors when compiled using:

g++ -o test.exe test.c

Errors:

test.c: In function 'int main()':
test.c:6:10: error: expected unqualified-id before 'new'
test.c:6:10: error: expected initializer before 'new'
test.c:7:32: error: invalid conversion from 'void*' to 'int*'
test.c:10:9: error: expected type-specifier before '=' token
test.c:10:11: error: lvalue required as left operand of assignment
test.c:12:36: error: expected type-specifier before ')' token