Yog-Sothoth's name was first mentioned in his novella 'The Case of Charles Dexter Ward' (written 1927, first published 1941). It goes on to be the driving force of the short story The Dunwich Horror, in which it fathers twin children with a human. The being is said to take the form of a conglomeration of glowing bubbles or spheres. It is later mentioned in At the Mountains of Madness as being the thing beyond the mountains that even the Elder Things fear.
It is heavily implied, if not outright stated, that Yog-Sothoth is omniscient, and is locked outside the universe, meaning he knows and can see all of space-time all at once, that there is no secret hidden from Yog-Sothoth.
Like many Lovecraftian gods, Yog-Sothoth has many different appearances throughout the various stories of the mythos, by various authors. However, there seems to common agreement that Yog-Sothoth visually manifests as a mass of glowing orbs, with eyes or tendrils in some versions, and in others simply the orbs.
"Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate. Past, present, future, all are one in Yog-Sothoth. He knows where the Old Ones broke through of old, and where They shall break through again. He knows where They have trod earth's fields, and where They still tread them, and why no one can behold Them as They tread."
—H. P. Lovecraft, "The Dunwich Horror"
Yog-Sothoth is called upon from time to time to assist mortals in performing occult deeds or rituals. The most notable instances of these events are recorded in The Case of Charles Dexter Ward and The Dunwich Horror.
One ritual involving Yog-Sothoth makes use of the "essential salts" of a deceased individual to resurrect them from the dead. The incantations involved with this are transcribed approximately as follows:
The incantation to put down the resurrected individual is the partial syllabic reverse of the first, and reads:
In one case involving the town of Dunwich, Yog-Sothoth is known to have been summoned for the purposes of impregnating a human female, who then gave birth to two partially-human children. The summoner was the head of the Whateley family, who was known to have stood on a hill in a circle of stones with the Necronomicon while shouting the name of Yog-Sothoth from the summit.
"Randolph Carter," IT seemed to say, "MY manifestations on your planet's extension, the Ancient Ones, have sent you as one who would lately have returned to small lands of dream which he had lost, yet who with greater freedom has risen to greater and nobler desires and curiosities. You wished to sail up golden Oukranos, to search out forgotten ivory cities in orchid-heavy Kled, and to reign on the opal throne of Ilek-Vad, whose fabulous towers and numberless domes rise mighty toward a single red star in a firmament alien to your earth and to all matter. Now, with the passing of two Gates, you wish loftier things. You would not flee like a child from a scene disliked to a dream beloved, but would plunge like a man into that last and inmost of secrets which lies behind all scenes and dreams. What you wish, I have found good; and I am ready to grant that which I have granted eleven times only to beings of your planet - five times only to those you call men, or those resembling them. I am ready to shew you the Ultimate Mystery, to look on which is to blast a feeble spirit. Yet before you gaze full at that last and first of secrets you may still wield a free choice, and return if you will through the two Gates with the Veil still unrent before your eyes."
-Yog-Sothoth addressing Randolph Carter, Through the Gates of the Silver Key