Saarthal is an ancient Nordic ruin located southwest of Winterhold (and to the northwest of the Shrine of Azura) that was once the earliest site of civilization in Skyrim, built during the time of Ysgramor himself. The city would suffer a major attack at the hands of the "Snow Elves" (well before they turned into the Falmer) in an event which would become known as the Night of Tears.
The College of Winterhold is a conducting an excavation of the ruin, and if the Dragonborn has been admitted into the College, s/he will be asked to participate as part of a field trip... as well as spark a sequence of events that will determine the fate of the College, and possibly the rest of Skyrim itself.
By the Fourth Era, Saarthal houses an extensive ruin structure now guarded by Draugr, Restless Draugr and Draugr Scourges, and is also where the mysterious Jyrik Gauldurson was sealed away following what appears to be a terrible act of treason. Gauldurson of course is now a very powerful Draugr himself, guarding a mysterious artifact which the College will come to call the Eye of Magnus.