Well it's similar to voodoo and obeah because people do get possessed in this religion, it deals with many deities and it's African in origin but it's different how it's practiced. IFA and Santeria usually deal with initation where as Voodoo and Obeah people can do it at will if they know what they are doing and they are a lot more amoral in the way they are practiced. A way to look at it is IFA is a religion from the Yoruba in Nigeria and it involves a supreme god or Orisha known as oludamare and others who work for him basically. Santeria on the other hand is a Cuban tradition of IFA practiced here in the diaspora. It was brought over her by way of slavery, basically slaves were taken from the yoruba people and brought to cuba and they kept their practices going in secret. This was done in a way that preserved the religion and it involved hiding the orisha's behind catholic saints. An Example is Oshun the godess of love was hid by the catholic Caridad del Cobre or Our Lady of Charity in english. They basically looked at the catholic saints and the traits of them, found similarities and hid their deities behind them. There are some that still practice this way and are strong catholics but it's not the only way.
Oshun
(Yes that's beyonce)
Caridad Del Cobre
Others who preserved the tradition are the shango baptist in Trinidad who mix what they do with baptist Christianity, the candomble of Brazil and a few others but some people in Nigeria still practice IFA as well.
If you want to learn more you could read my post here
http://www.thecoli.com/posts/23877547/
Or visit my thread and watch some of the videos in the OP
The African Traditional And Diasporic Religions Thread (Santeria, IFA, 21 Divisions, Sanse + etc)
I don't know what's Green magic but I guess plants.