According to some comments on the question Are women allowed to be Hindu sanyasis/monks? Some people believe that Devdasi and Sanyasin are near synonyms, and others that they are very different.
How are Devdasi and Sanyasin different?
Hinduism Stack Exchange is a question and answer site for followers of the Hindu religion and those interested in learning more about Hinduism. It only takes a minute to sign up.Sign up to join this community
In Tamil, Devdasis are called "Dev-adiyal".
Adiyal means servant.
In fact, Dasi means nurse / servant / attendant.
Dev-dasis/Dev-adiyals were basically attendants of the lord. Generally it can be understood that temples adopted and took care of poor and orphan children in the neighbouring community. These children grew up in the temple, attending to temple functions.
As time went by, rich and powerful men formed relationships with these women or abused them, slowly turning them into prostitutes, and undesirables of the society.
The word Devdasi or Devadiyal is a vulgar curse word now, used to curse at women. At one time it wasn't so; it was a term of respect.
Then it meant "Lord's Attendant". Now it sadly means "Prostitute".
Source: My knowledge of Tamil and oral historical tradition
Sanniyasis are hermits.
Saṃnyāsa in sanskrit means renunciation. From that you get a Sanniyasi - someone who threw away all worldly attachments.
Traditionally Sanniyasis retreat into forests, caves and similar secluded areas to meditate.
Source: Knowledge of Sanskrit and oral historical tradition
Devadasi system was the one most prevalent in southern India and parts of Odisha. Devadasis were young girls who were "dedicated" to the worship of deities. They had to gain expertise in arious classical dance forms like Bharatnatyam and Odissi. They were thought to be MARRIED TO THE LORD himself. However they had various kings and influential people as their patrons. Information regarding Devadasis can be viewed at http://en.wikipedia.org/wiki/Devadasi.
Many researchers have called Devadasis a "cursed" community. It is because many of the devadasis have themselves confessed of being "dedicated to a life of sex work" in the name of religion. The Devadasi system continues in many parts despite being declared illegal in 1988. For details visit: http://www.theguardian.com/lifeandstyle/2011/jan/21/devadasi-india-sex-work-religion
However, sanyasinis are those women who have left GARHASTHYA to take up "sannyasa" or have proceeded to the same right after Brahmacharya without even entering into Garhasthya. Consult http://en.wikipedia.org/wiki/Sannyasa and http://www.yogamag.net/archives/1979/gjuly79/qualsan.shtml
Unlike the devadasis, the sanyasinis maintain celibacy. They are dedicated not merely to the deity but play a role in the betterment of the society too. There has been no declaration of sannyasa being illegal till date.
Devadasi are Sanyasini with specialization in Art.
If we look at both Shaiva and Vaishnava traditions, they have different forms of worship to Lord. One of them is through 64 forms of Art. These artists perform only for their God and serve the community through the temple.
In Shaiva tradition they are called "RudraKanya" and in Vaishnava tradition they are "DevaDasi". They are Sanyasinis, but specialize in Arts and temple maintenance. Additionally, they also serve the community as a teacher as well in the space of motherhood.
PS: I am not aware of if Shakta traditions have something similar