In Islam, a Maturidi is one who follows Abu Mansur Al Maturidi's theology, which is a close variant of Ash'ari school of thought and the codifying of the beliefs of traditional Sunni Islam as practiced since the time of the Prophet Muhammad. The Maturidis, Asharis and Atharis are all part of the majority of Muslims, the Sunnis. They only differ on minor arbitrary points in belief. One of the earliest and most basic texts on Islamic theology, al-Aqeedah al-Tahawiyyah (The Tahawi Doctrine of Belief), is a Maturidi text.
Points which the Maturidis differ from the Ash'aris are the nature of belief and the place of human reason. The Maturidis state that belief (iman) does not increase nor decrease but remains static; it is piety (taqwa) which increases and decreases. The Ash'aris say that belief does in fact increase and decrease. The Maturidis say that the unaided human mind is able to find out that some of the more major sins such as alcohol or murder are evil without the help of revelation. The Ash'aris say that the unaided human mind is unable to know if something is good or evil, lawful or unlawful, without divine revelation.
Another point where Ash'aris and Maturidis differ is divine amnesty for certain non-Muslims in the afterlife. The Ash'ari view of Imam al-Ghazali says that a non-Muslim who was unreached by the message of Islam or was reached by it in a distorted fashion, is not responsible for this in the afterlife. The Maturidi view states that the existence of God is so obvious, that one who has intellect and time to think (not the mentally retarded etc.) and was unreached by the message of Islam and does not believe in God will end up in the hellfire, and divine amnesty is only availible to those non-Muslims who believed in God and were unreached by the message.