Definition: Noun is a part of speech that represents people, animals, places, physical objects, or abstract ideas.
In Spanish, as in many other languages, nouns have grammatical gender (feminine or masculine) and number (singular or plural). The gender and the number of a Spanish noun are indicated by its article (also called determiner): la/las or una/unas for feminine singular/plural and el/los or un/unos for masculine singular/plural nouns.
Example:
feminine (sing.) nouns: una pelicula (a movie), la cancion (the song)
feminine (pl.) nouns: unas películas (some movies), las canciones (the songs)
masculine (sing.) nouns: un país (a country), el árbol (the tree)
masculine (pl.) nouns: unos países (some countries), los árboles (the trees)
Note: It's best to learn nouns with their corresponding article. Though there are some rules for predicting the gender of a noun by its ending, there are many exceptions to those rules.