I think you mean "articles" "a" and "an". They are both indefinite articles ("the" being a definite article). You use "an" when the words starts with any other vowel than 'y' (a,e,i,o,u). Use "a" when the word begins with a consonant. To see why this is so, try saying "a apple". You have to stop vocally between "a" and "apple", usually in the back of the throat, which is clumsy. "An apple" is much simpler to say and make yourself understood.

Hi... Correction : "She is wearing an orange skirt."...