Orishas are manifestations of the supreme divinity Olodumare in the traditional religions of the Yoruba people in West Africa (YTR). They are also known as Voduns in the Dahomean (now Benin) religion (also known as Vodun). To diminish their beliefs, slave traders and colonisers used to tell that African people were polytheists (worshipping several gods).
These traditional African spiritualities were brought to the Americas via the Atlantic slave trade. You can find the same divinities in Brazilian Candomblé and Umbanda, Cuban Santeria, Haitian Vodou, Trinidad Shango etc...
Orishas represent forces of Nature. They provide a connection between the supreme divinity and humans (as there's no way for humans to grasp this unfathomable power of creation).
It is said that there is an infinite number of Orishas. These are a few of the principal ones.