The Red Book: Valaquenta: The Maiar and The Enemies

If you don't want to dig into the quotes, here is a simple summary.

The Maiar are servants of the Valar, set on Arda to aid them in their works. They are also susceptible to corruption, and many became servants of Melkor.
The Istari, or wizards, are a division of the Maiar.
Balrogs are also Maiar, and are called Valaraukar.
Sauron is, in fact, a Maia*. As Melkor was among the greatest of the Valar, Sauron was among the greatest of the Maiar.

The Maiar

In this section of Silmarillion, Tolkien describes the Maiar as spirits, the 'people of the Valar' their 'servants and helpers'. It is not known how many there were, and they rarely showed themselves to men or elves. They were not slaves, however, and had free will. Nor were they incorruptible.

“With the Valar came other spirits whose being also began before the World, of the same order of the Valar but of less degree.”

“Chief among the Maiar of Valinor whose names are remembered in the histories of the Elder Days are Ilmarë, the handmaid of Varda, and Eönwë, the banner-bearer and herald of Manwë […] But of all the Maiar Ossë and Uinen are best known to the Children of Ilúvatar.”

Uinen and Ossë

Ossë is sort of the assistant to Ulmo, and is master of the sea shores of Middle-earth. He does not like deep waters, but prefers costs, isles, and wind “for in storm he delights, and laughs amid the roaring of the waves.” I feel like we would get along.

Uinen, called also the 'Lady of the Seas', is his spouse.
“All creatures she loves that live in the salt streams, and all weeds that grow there; to her mariners cry, for she can lay calm upon the waves, restraining the wildness of Ossë. The Númenóreans lived long in her protection, and held her in reverence equal to the Valar.”

During Arda's forming Melkor, unable to subdue the sea, drew Ossë to his side by promising him all the realm of Ulmo. “So it was that long ago there arose great tumults in the sea that wrought ruin to the lands.”

Fortunately Uinen was able to restrain Ossë and bring him to Ulmo, who then pardoned him. Ossë thereafter remained faithful, though “his delight in violence never wholly departed from him, and at times he will rage his willfulness without any command from Ulmo his lord.”

Melian

Melian is a Maia who served Estë and Vána; before coming to Middle-earth she lived for some time in Lórien tending the flowering trees in Irmo's gardens.

“Nightingales sang about her wherever she went.”

“Of Melian much is told in the Quenta Silmarillion. But of Olórin that tale does not speak; for though he loved the Elves, he walked among them unseen, or in form as one of them, and they did not know whence came the fair visions or the promptings of wisdom that he put into their hearts. In later days he was the friend of all the Children of Ilúvatar, and took pity on their sorrows; and those who listened to him awoke from despair and put away the imaginations of darkness.”

Olórin (Gandalf):

The Istari:

The Istari are Maiar that were sent to assist the people of middle-earth against evil (Sauron and Morgoth). In Middle-earth they are known as wizards. In Valaquenta, only Olórin is mentioned, and so we shall talk about the others when we come to them in the book. He is also known as Gandalf, and I shall give him a whole post all to himself next week.

"Wisest of the Maiar was Olórin. He too dwelt in Lórien, but his ways took him often to the house of Nienna, and of her he learned pity and patience."

Sauron

Sauron was one of Aulë's Maiar, and became Melkor's sidekick. He is also called Gorthaur the Cruel.

“In all the deeds of Melkor the Morgoth upon Arda, in his vast works and in the deceits of his cunning, Sauron had a part, and was only less evil than his master in that for long her served another and not himself. But in after years he rose like a shadow of Morgoth and a ghost of his malice, and walked behind him on the same ruinous path down into the Void.”

Morgoth

Not a Maia, but one of the Valar. Since I am also covering the section titled “The Enemies” in Valaquenta, I had to include this amazing quote.

“From splendor he fell through arrogance to contempt for all things save himself, a spirit wasteful and pitiless. Understanding he turned to subtlety in perverting to his own will all that he would use, until he became a liar without shame. He began with the desire of Light, but when he could not possess it for himself alone, he descended through fire and wrath into a great burning, down into Darkness. And darkness he used most in his evil works upon Arda, and filled it with fear for all living things.”

I have no words.

Valaraukar

Many of the Maiar joined Melkor at the beginning, and still more he drew to his side with lies and bribes. Among them were the Balrogs. Keep in mind that they don't start out with a physical form that we can see, but many of them seem to choose their paths and settle down over time into a certain set of skills.