When you substitute a pronoun for a noun, i.e. when you say Онђ здљсь 'It is here' instead of Газљта здљсь 'The newspaper is here', you have to choose from among three pronouns: ћн 'he, it', онђ 'she, it', and онћ 'it'. Nouns fall into 3 classes, called gender classes, depending on which pronoun is required. The names for the three classes are: masculine (ћн), feminine (онђ), and neuter (онћ).
masculine: Гдљ стакђн? Вћт ћн. Where's the glass? There it is.
  Гдљ учќтель? Вћт ћн. Where's the teacher? There he is.
  Гдљ Сђша? Вћт ћн. Where's Sasha? There he is.
feminine: Гдљ рџчка? Вћт онђ. Where's the pen? There it is.
  Гдљ Мђша? Вћт онђ. Where's Masha? There she is.
neuter: Гдљ письмћ? Вћт онћ. Where's the letter? There it is.
These three gender classes correspond quite closely to the first three declension classes: generally speaking, #-declension nouns are masculine, o-declension nouns are neuter, and a-declension nouns are feminine.
There are exceptions to this, the most important of which is this: the gender of a nickname (a-declension) is masculine if the noun refers to a male. The same is true of nouns such as пђпа 'Dad,' дїдя 'uncle,' and дљдушка 'Grandpa.'