vs. lui / ei
"al/a/ai/ale" is an agreeing possessive article; "lui/ei" are possessive pronouns or prepositional genitive markers in other constructions.
cartea lui Andrei / cartea al lui Andrei
Andrei's book / the book of Andrei
vs. a
"a" is the feminine singular form corresponding to masculine singular "al".
un prieten al meu / o prietenă a mea
a friend of mine / a female friend of mine
vs. ai / ale
"ai" and "ale" are plural forms used with masculine and feminine plural nouns respectively.
frații mei / surorile mele
my brothers / my sisters