Some nouns are masculine (male / boy). Some nouns are feminine (female / girl). Some other nouns are neutral because they are used for both the masculine and the feminine. Let's take a look at some examples: 

          MALE          FEMALE          NEUTRAL
   boy              girl                  baby 
         nephew         niece               cousin 

It is good to get familiar with the following: 

         MALE                       FEMALE
          man                          woman 
          king                          queen 
          lord                           lady
          uncle                        aunt
          grandma                 grandpa
          actor                        actress
          waiter                      waitress
          father                       mother
          prince                      princess