There three declinations of nouns in Russian

To the 1st declination belong masculine and feminine nouns ending in -а, -я:

машина, песня, папа, мама

To the 2nd declination belong masculine nouns with null ending and neutral nouns ending in -о, -е:

компьютер, окно, море

To the 3rd declination belong feminine nouns with null ending (including -ь):

ночь, дочь, дверь