Leto is the daughter of the Titans Coeus and Phoebe. Her name is probably some variant of unseen or hidden. She is often depicted wearing a veil implying matronly modesty as one of her main attributes. In Greek myth Leto is the goddess of motherhood and modesty and is, along with her divine twins, a protector of young children.
Leto is the mother of Artemis and Apollo, whose father is Zeus. As with most of Zeus’ affairs outside of his marriage to Hera, Hera was jealous of Leto and chased her from country to country as she tried to rest to give birth to her twins. She finally found rest on the island of Delos. This is why Delos is sacred to Apollo and Artemis.
Although often pictured among the Olympian gods, playing the lyre, she is not generally considered one of the 12 Olympian gods.
Leto is one of a multiplicity of female moon and fertility goddesses that were adopted by the Greeks as they came in contact with other cultures, particularly those in Asia Minor (now Turkey). Leto was the primary goddess of Lycia in Anatolia. She is also strongly associated with the island of Kos, which was considered her birthplace.
When being worshipped, Leto is most often associated with her more important children. Her most important cultic centers seem to be Kos, Lycia and Phaistos on Crete, where she seems to be the goddess of a mystery cult. But she had temples and altars throughout the Greek world.
Around Athens in Attica; Leto had a temple in which the local sailors had dedicated weapons taken from the Persian during the Persian War.
The Greek travel writer Pausanias in his Description of Greece written in the 2nd Century AD makes many mentions of Leto’s cultic sites.
In Megara was a sanctuary to Apollo with a “noteworthy statue” of Leto “made by Praxiteles.” (Pausanias; 1. 44. 2)
In Argos, “is the sanctuary of Leto; the image is the work of Praxiteles.” Also near Argos was “a sanctuary of Artemis Orthia” with “white-marble images of Apollon, Leto and Artemis, which they say are works of Polykleitos.” (Pausanias; 2. 21. 9 and 2. 34. 5)
In Sparta: “On their market-place the Spartans have images of Apollon Pythaios, of Artemis and of Leto.” (Pausanias; 3. 11. 9)
At Therai , also in Lacaedaimonia there had “been dedicated [statues of] Leto.” (Pausanias; 5. 17. 3)
At the city of Mantineia in Arcadia: “The Mantineans had a sanctuary of Leto and her children, and their images were made by Praxiteles.” (Pausanias; 8. 9. 1)
At Tanagra in Boeotia: “are images of Artemis and Leto.” And also was a temple to “Apollo joined [by] Artemis and Leto.” (Pausanias; 9.20.1 and 9.22.1)
Pausanias, Description of Greece at http://www.perseus.tufts.edu/hopper/text?doc= Perseus:text:1999.01.0160&redirect=true