What is Ash'arism? And is it considered one of the doctrines of Ahlus-Sunnah wal-Jama'ah?

The Ash'aris are a sect attributed to Abu al-Hasan al-Ash'ari. They are considered among the Ahl al-Sunnah (People of the Sunnah) in matters where they concur with the Sunnah, such as matters of transmitted knowledge (sam'iyyat) and many aspects concerning prophethood (nubuwāt). However, in matters of divinity (ilāhiyyāt), they differ from much of what the righteous predecessors (Salaf al-Salih) were upon.