c++ - How can I use std::maps with user-defined types as key?

I'm wondering why I can't use STL maps with user-defined classes. When I compile the code below, I get the following cryptic error message. What does it mean? Also, why is it only happening with user-defined types? (Primitive types are okay when they are used as key.)

C:\MinGW\bin..\lib\gcc\mingw32\3.4.5........\include\c++\3.4.5\bits\stl_function.h||In
member function `bool

std::less<_Tp>::operator()(const _Tp&,
const _Tp&) const [with _Tp =
Class1]':|

C:\MinGW\bin..\lib\gcc\mingw32\3.4.5........\include\c++\3.4.5\bits\stl_map.h|338|instantiated
from `_Tp& std::map<_Key, _Tp,
_Compare, _Alloc>::operator[](const _Key&) [with _Key = Class1, _Tp = int, _Compare = std::less, _Alloc = std::allocator >]'|

C:\Users\Admin\Documents\dev\sandbox\sandbox\sandbox.cpp|24|instantiated
from here|

C:\MinGW\bin..\lib\gcc\mingw32\3.4.5........\include\c++\3.4.5\bits\stl_function.h|227|error: no match for 'operator<' in '__x <
__y'| ||=== Build finished: 1 errors, 0 warnings ===|

#include 
#include