A noun is a word that identifies someone or something - a person, an animal, a thing, an abstract notion etc.
женщина - woman
кошка - cat
дом - house
успех - success
In English, nouns have two forms - singular and plural. In most cases, all you need to do is add an -s or -es to make the plural form, i.e. dog - dogs, car - cars, bus - buses.
In Russian, things are a bit more complex.
Nouns have 2 forms (also referred to as number) - Singular and Plural Form
Nouns can either be Masculine, Feminine, or Neuter in Gender
Nouns belong to the First, Second, or Third Declension
Nouns have six cases - Nominative, Genitive, Dative, Accusative, Instrumental, and Prepositional cases
Please refer to the following topics to learn everything about Russian nouns in more detail:
Singular and Plural Form of Nouns
Gender of Nouns
Cases of Nouns