Mullah ( ملا) is a Muslim learned in Islamic theology and sacred law. The title, given to some Islamic clergy, is coming from the Arabic word mawla, meaning both 'vicar' and 'guardian.' In large parts of the Muslim world, particularly Iran, Afghanistan, Turkey, Central Asia and the Indian subcontinent, it is the name commonly given to local Islamic clerics or mosque leaders.
It is primarily understood in the Muslim world as a term of respect (a learned man), though a fairly common usage on the internet includes abuse; grouping Mullah with terrorist, bigot, fanatic.
Ideally, a trained Mullah will have studied Islamic traditions (hadith), and Islamic law (fiqh). They are often hafiz, i.e. have memorized the Qur'an. However, uneducated villagers often recognize a literate Muslim with a less than complete Islamic training as their "mullah" or religious cleric. Mullahs with varying levels of training lead prayers in mosques, deliver religious sermons, and perform religious ceremonies such as birth rites and funeral services. They also often teach in a type of Islamic school known as a madrasah. This triumvirate of knowledge is applied mostly in interpreting Islamic texts (ie. the Quran, Hadiths, etc.) for matters of Shariah, ie Islamic law. Mullah's are often shown in western media as being extreme; it can be agreed that every muslim differs in the strenuousness of his/her practice, and belief in the teachings of Islam.
The term is seldom used in Arabic-speaking areas, where its nearest equivalent is shaykh (implying formal Islamic training), imam (prayer leader; not to be confused with the Imams of the Shiite world), or `ālim (plural `ūlamā') (scholar; see ulema). In the Sunni world, the concept of "cleric" is of limited usefulness, as authority in the religious system is relatively decentralized.
The term is frequently used in English, although English-speaking Muslim clergy rarely call themselves mullahs. It was adopted from Urdu by the British rulers of India and subsequently came into more widespread use.