Victoria, one of my readers from Germany, asked a question about the relationship between the lwa and the Orisha. This is an interesting topic and one that frequently comes up online and elsewhere. What follows is my attempt at giving a complex question a relatively simple answer.
There's a tendency to see "African culture" as an enormous monolithic civilization. In fact, Africa is a continent which contains an estimated 2,000 languages: Nigeria alone has an estimated 250. There are several important cultural groups within Western and Central Africa, the area most profoundly effected by the Middle Passage. The deities of the Fon/Ewe people of Daome (in modern-day Togo and Benin) are the main spirits served as the "Rada Lwa." (A group which takes its name from the slave port of Arara, located in Ouidah -- a name which is reflected in "Damballah Wedo" or "Damballah of Ouidah").
Another powerful group within the region was the Yoruba or Nago peoples, most of whom resided in and around the area of modern-day Nigeria. One of their major deities was the blacksmith/warrior Gu, lord of iron and warfare. In Haiti he was served as Ogou, while Cubans call him Ogoun and Brazilians Ogum. In Vodou many Ogou songs praise him as "Olisha Nago" or "Nago Orisha." In fact, all the spirits who originate in Yorubaland are part of the "Nago Nation." That includes Ogou Shango (Chango) and Ogou Batala (Obatala), a lwa served with white and red who resembles the "warrior road" of Obatala, Obatala Ayaguna.
Many people compare Obatala and Damballah because both have white as their primary color, both are known for their even tempers and both are very clean spirits who dislike dirt, smoke and alcohol. But Damballah is actually a Fon/Ewe spirit, the great serpent Da, while Obatala is from Yoruba culture. It's like conflating Osiris and Jesus: both are connected with death and resurrection but the stories connected with them and their cultural roles are quite different.
Erzulie Freda takes her name from Aziri, a river in Fon/Ewe territory. In Nigeria there is a goddess honored on the Osun River. So while Erzulie Freda and Oshun are both African river spirits and both share some common interests, they originate in different places and cultures. A lot of Vodouisants believe that Freda and La Sirene are sisters who are both married to Met Agwe, the Lord of the Oceans, but they are definitely see as different spirits.
Vodou originated in Haiti and its spirits come largely from the Fon/Ewe area with a sizeable influence of Central African Bantu/Congo spirits and practices. Lukumi/Santeria comes from Cuba and is almost exclusively derived from Yoruba traditions: it is seen as "Las Reglas de Ocha" while Congo-derived practices like Palo Mayombe are "Las Reglas de Congo."
I will be posting this to my lists, where there are several members who are initiated in Palo or Lukumi, and will be sure to include any of their corrections or comments. But this should give you an idea of some of the distinctions between the two.