She is my younger sister.
彼女にこの本をあげた。
I gave her this book.
Refers specifically to a romantic partner. Often used with possessive 私の or in context where the relationship is clear. Compare with 彼氏 (boyfriend).
See also: 彼氏
昨日、彼女と映画を見に行った。
Yesterday I went to see a movie with my girlfriend.
彼は新しい彼女ができたらしい。
It seems he has a new girlfriend.
I got a girlfriend last month.
He has been dating his girlfriend for three years.
He broke up with his girlfriend last week.
This is my girlfriend.