Articles are a type of determiner used to indicate the type of reference being made to a noun. In English, the articles are "the", "a" and in certain contexts "some".
lo
the
masculine singular before /z/ or /s/ + consonant definite
il
the
masculine singular definite
uno
a
masculine before /z/ or /s/ + consonant indefinite
gli
the
masculine plural before vowel, or /z/ or /s/ + consonant definite
degli
some
masculine before vowel, or /z/ or /s/ + consonant indefinite
un
a
masculine indefinite
dei
some
masculine indefinite
i
the
masculine plural definite
delle
some
feminine indefinite
una
a
feminine indefinite