The "definite articles" are those that specify the gender and number of the noun they precede. They are called definite because they refer to people or things known to the interlocutors, for example, if I say: "The dog has not eaten", the definite article "the" tells us that it is something or someone masculine and singular, but also, both the speaker and the listener know which dog we are referring to. The "definite articles" are: "el" masculine-singular, "los" masculine-plural, "la" feminine-singular, "las" feminine plural. (watch video)
"Indefinite articles" are those that specify the gender and number of the noun they precede. They are called indefinite because they refer to non-specific people or things. For example, if I say: “A dog ate my chicken,” the indefinite article “un” tells us that it is a dog that we have not seen before. The "indefinite articles" are: “un” masculine-singular, “unos” masculine-plural, “una” feminine-singular, "unas" feminine-plural. (watch video)