Most likely, it's in a different case.

For example, "книга" and "книгу" both mean "book", but "книга" is in the nominative case (subject) and "книгу" is in the accusative case (direct object).

Also, if a word ends in "а" in it's nominative form, it is a feminine word. And if it ends in "й" in it's nominative form, it is masculine.