Tonight we had a lively girls' talk session.
That shop sells clothes for girls.
ガール is the singular form for one girl, whereas ガールズ is the plural form referring to a group of girls or women.
The everyday counter for people: 一人 (ひとり), 二人 (ふたり), 三人 (さんにん).
More formal than 人 — used for headcounts in reservations, staff lists, and announcements (5名様).
girl's bar; small bar with female staff
all-female band; girl band
girl group
girl talk
girls' love; GL; yuri media
Borrowed from English 'girls', with the plural 's' retained in the Japanese form.