Vietnam War Collectibles

I understand that the Americans were fighting the Vietcong (who wanted to make the south communist?), but why were the fighting the ARVN (who were from the south) too? Weren't they supposed to be working with the south vietnamese to stop communism from spreading throughout Vietnam, hence why they used pacification to keep on the side of the south vietnamese?

I only need it explained briefly but there are too many names and armies etc and I'm getting in a right muddle!

~Mystic.
Ah! I get it, thank you so much!