*[Rich][1]* and *[poor][2]* are adjectives.  
>‘a rich and famous family’  
 
With the definite article, they become phrasal nouns: "*the rich*" and "*the poor*".  
>‘every day the split between the rich and the poor widens’  

That is, **those who are rich** and **those who are poor**.  

  [1]: https://en.oxforddictionaries.com/definition/rich
  [2]: https://en.oxforddictionaries.com/definition/poor