C++ :: Simple Class Usage Compiler Error

Full disclosure: this is an exercise from "Sams Teach Yourself C++ in 24 Hours" by Jesse Liberty and Rogers Candenhead. This refers to Chapter 9 (Hour 9 Activity 1)

I created a class called Point, in Point.h

I created a class called Rectangle in Rectangle.h and Rectangle.cpp

If I create an int main() function in Rectangle.cpp (that includes Rectangle.h), I can compile Rectangle.cpp and run the resulting program. Fine.

Question:

I create a separate file called main.cpp. I include Rectangle.h. But now the compiler complains.

Code:
$ g++ main.cpp -o main
/tmp/cc38JIph.o: In function `main':
main.cpp:(.text+0x26): undefined reference to `Rectangle::Rectangle(int, int, int, int)'
main.cpp:(.text+0x32): undefined reference to `Rectangle::getArea() const'
collect2: ld returned 1 exit status If I can create a class in Point.h and use it in Rectangle.h, why can I not just use Rectangle in main.cpp?

And the files, of course:

file: main.cpp
Code:
#include <iostream>
#include "Rectangle.h"
using std::cout;
using std::endl;

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