[Valmont]

In general, the type that is being compared needs to be able to be compared with the same type. For example if you would like to sort integers then at one moment a comparison is needed like:

if( FirstInt < SecondInt)
{
   //Do stuff.
}

This snippet implies that the type of FirstInt and SecondInt can be compared with eachother.
The class or type they represent needs to be a comparable class or type.

Suppose one wants to sort humans. Such a definition doesn't make much sense. The next refinement is that I may want to sort people by some sort of property they have. This looks reasonable. The whole trick is now to define a class CHuman that is comparable. The same goes for cars, houses, services and whatever you can think of.

Conclusion:
Study the ways of making comparable classes.