The Muses are the inspirational goddesses of literature, science and arts, and were considered the source of knowledge embodied in the poetry, lyric songs and myths that were related orally for centuries in Greek culture.
In total, they numbered nine: Calliope (epic poetry), Clio (history), Erato (love poetry), Euterpe (music, song & lyric poetry), Melopomene (tragedy), Polyhymnia (hymns), Terpsichore (dance), Thalia (comedy), Urania (astrononmy).
The cult of the Muses was also closely connected to Apollo.
The sanctuary consisted of a theatre, which has been dated to the end of the 3rd c. BCE, or the beginning of the 2nd C. BCE, built for the musical and theatrical games held in the valley during the festival of the Muses (Mouseia). Spectators sat on seats cut into the mountain slope, but only the first row (the prohedria) was marble.
The temple or altar of the Muses, a small rectangular building, has been dated to the 3rd c. BCE. After its disuse, the church of Ayia Triada was built on the ancient foundations, which were revealed after the church's demolition.
A long Ionic stoa (96.70m in length) on the west of the temple was dated to the 3rd c. BCE, which housed the votive offerings to the Muses. Originally it had an internal wall and rooms which were later replaced by a colonnade in the Corinthian order.
Around the temple stood statues of the nine Muses, works of the poet and sculptor Onestos. These orignally stood in a single group, and their bases were discovered during the excavations. On the five most well-preserved bases, inscribed names of the Muses and epigrams were found.
Constantine removed the statues of the Muses to Constantinople, and so it is not coincidental that a chapel was built to him above the Muses' sanctuary.1
In this sanctuary it is not just the Muses who are worshipped. Some inscriptions, dedicated to Herakles Charops, lead us to assume that he was also an important figure. Pausanias confirm this through the cult aition which is helpful to explain why Herakles had a virgin priestess. Dedications and inscriptions help us also to define the name of the other divinity worshipped in the sanctuary; in the list we can add Demeter and Hermes.
“After the classical period, the only cult activity at the sanctuary which was not somehow related to the Mouseia seems to have been an annual sacrifice to the Muses.”2
The first dedication that we know was made from Hesiod who dedicated to the Muses the tripod that he won at the funeral games for Amphidamas and which he placed on the Helikonian, on the spot where they had inspired him. After that, lots of statues and votive donations were made to the sanctuary, especially statues which represented the group of Muses. This use continued until the end of the fourth century A.D. and the big number of donations have leaded scholars to assume that the Museia was the place from which our concept of “museum” comes. We know also that there was a statue dedicated to Herakles, which Pausania mentioned. Other dedications consist in inscriptions on statue bases, especially for Demeter and Hermes. Hermes seems one of the most worshipped, next to the Muses; in fact, to him was also donate a statue, made by Lysippos, in which the god is represented fighting with Apollo for the lyre. It seems also that Philetarios had donated to Hermes a land which income were destined to the sanctuary itself. There are also some depicted fragments with Hermes on, that seem have been part of a bigger composition. In the end, from early first century B. C., seems that people of Thespiai had set up statues of important Romans, in the grove of the Muses.
We know that Thespiai used to organize the Panhellenic festivals and the relative contests in honour of the Mouses and Eros, celebrated by the Thespians on Mt. Helicon, at the end of every four years. We have records about it in the works of several ancient writers, as Pausanias or Plutarch.
The main use for the Sanctuary of the Muses at Thespiai was, of course the Mouseia and, after the 1st Century BC, the Great Kaseireiea. There were, however, various other events that took place, for example it seemed to hold significance for the inspiration of literary figures. Hesiod claimed to have been inspired here to wrote his Theogony, writing
“So said the ready-voiced daughters of great Zeus, and they plucked and gave  me a rod, a shoot of sturdy laurel, a marvellous thing, and breathed into me a divine voice to celebrate things that shall be and things that were aforetime; and they bade me sing of the race of the blessed gods that are eternally, but ever to sing of themselves both first and last.”
Furthermore, Callimachus once dreamt of being carried to Mount Helicon where he, too, was then instructed by the Muses (Aitia). Finally, Pausanius writes that there were goats grazing in the surrounding area (see photos below), which was supposed to be very fertile. He talks about an abundance of wild strawberry bushes and suggests that people also benefited from the high quality natural resources -- the food was supposed to make snake venom less deadly. (9.28.1)