Definite Article

Definite Article
English
I read the book you recommended.
(a specific thing - the book you recommended)
I ate the apple you gave me.
(a specific thing - the apple you gave me)
Note: In English there is no change to the definite article regardless of the number of items. The dog is barking. The dogs are barking.

Spanish
In Spanish, as in English, a definite article is used before a noun when referring to a specific person, place, animal, thing or idea.
Comí la manzana que me diste.
I ate the apple you gave me.

In Spanish the definite article is also used when speaking in general terms.
Me gustan los gatos pero odio los perros.
I like cats [in general], but I hate dogs [in general].

In Spanish the article must match not only the gender of the noun, but also must agree in number. In other words there are masculine and feminine definite articles and masculine plural and feminine plural definite articles.
El - indicates that the noun is masculine in gender and singular in number.
La - indicates that the noun is feminine in gender and singular in number.
Los - indicates that the noun is masculine in gender and plural in number.
Las - indicates that the noun is feminine in gender and singular in number.

Note: There are some exceptions to the above rule. Generally singular Spanish nouns that start with a stressed 'a' will take the definite article 'el'. This is done for pronounciation purposes, but the noun remains feminine. There are some other exceptions, but these are too minor to discuss here.