The first to point out would be that the Manga has much more detailed images. The storyline in the Manga is much more Mature and much deeper. The Manga contains suicide, and plenty of violence (much more compared to the Anime). The monsters/youmas/daimons/etc, are drawn in much greater detail as well. They are far more scary in the Manga. However nothing in the Manga would scare anyone over the age of 10.
The Manga is split into 18 Volumes or "Tankubon" there are 52 Acts in the 18 Tankubon. Each set of Tankubon form one part of the series...
Specials:
Tankubon/Vol. 10: Chibiusa's Pictorial Diary
Tankubon/Vol. 11: Kaguya Hime no Koibito (Kaguya Hime's Lover)
Tankubon/Vol. 11: Memories of Casa Blanca
Tankubon/Vol. 13: Makoto's Depression
Tankubon/Vol. 13: Ami's First Love
Tankubon/Vol. 13: Rei and Minako's Battle at the Female School
Tankubon/Vol. 15: Chibiusa's Pictorial Diary
Tankubon/Vol. 17: Chibiusa's Pictorial Diary
Tankubon/Vol. 01 - 04, Acts 01 - 3: Sailor Moon
Tankubon/Vol. 04 - 07, Acts 13 - 23: Sailor Moon R
Tankubon/Vol. 07 - 10, Acts 24 - 33: Sailor Moon S
Tankubon/Vol. 12 - 15, Acts 34 - 42: Sailor Moon SuperS
Tankubon/Vol. 16 - 18, Acts 43 - 52: Sailor Moon Sailor Stars