Ghazw (Razzia) (plural ghazawāt) is an Arabic word meaning an armed incursion for the purposes of conquest, plunder, or the capture of slaves and is cognate with the terms ghāziya and maghāzī. In pre-Islamic times it signified the plundering raids organized by nomadic Bedouin warriors against either rival tribes or wealthier, sedentary neighbors. In English language literature the word often appears as razzia, deriving from the French word razzier (rezzou) which entered the language at the time of the French colonization of North Africa, and which is itself a transliteration of the colloquial Arabic word ghazya.



Ghazi Warrior
For the ghāzīs in the marches, it was a religious duty to ravage the countries of the infidels who resisted Islam, and to force them into subjection. 
Cambridge History of Islam, p. 283 
Also:

After the conquests had come to an end, the legal specialists laid down that the caliph had to raid enemy territory at least once a year in order to keep the idea of jihad alive. 
Peters, Jihad in Classical and Modern Islam: A Reader, p. 3 
The ghāzī warrior dates to at least the Samanid period, where he appears as a mercenary and frontier fighter in Khorasan and Transoxiana. Later, up to 20,000 of them took part in the Indian campaigns of Mahmud of Ghazni.

Ghāzī warriors depended upon plunder for their livelihood, and were prone to brigandage and sedition in times of peace. The corporations into which they organized themselves attracted adventurers, zealots, and religious and political dissidents of all ethnicities. In time, though, soldiers of Turkic ethnicity predominated, mirroring the Turkic rise to military and later political dominance throughout the Muslim world.

In the west, Turkic ghāzīs made continual incursions along the Byzantine frontier zone, finding in the (often co-ethnic) akritai their counterparts. After the Battle of Manzikert these incrusions intensified, and also saw the ghāzī corporations coalesce into semi-chivalric fraternities, with the white cap and the club as their emblems. These organizations were fluid, however, reflecting their popular character, and ghāzī warriors would jump between them depending upon the prestige and success of a particular emir. It was from these Anatolian territories conquered during the ghazw that the Ottoman Empire emerged, and in its legendary traditions it is said that its founder, Osman I, came forward as a ghāzī thanks to the inspiration of Shaikh Ede Bali.

In later periods of Islamic history the honorific title of ghāzī was assumed by those Muslim rulers who showed conspicuous success in extending the domains of Islam, and eventually the honorific became exclusive to them, much as the Roman title imperator became the exclusive property of the supreme ruler of the Roman state and his family.

The Ottomans were probably the first to adopt this practice, and in any case the institution of ghazw reaches back to the beginnings of their state:

By early Ottoman times it had become a title of honor and a claim to leadership. In an inscription of 1337 [concerning the building of the Bursa mosque], Orhan, second ruler of the Ottoman line, describes himself as "Sultan, son of the Sultan of the Gazis, Gazi son of Gazi... march lord of the horizons." The Ottoman poet Ahmedi, writing ca. 1402, defines a gazi as "the instruments of God's religion, a servant of God who cleanses the earth from the filth of polytheism... the sword of God." 
Lewis, The Political Language of Islam, pp. 147-148 (note 8) 
Because of the political legitimacy that would accrue to those bearing this title, Muslim rulers vied amongst themselves for preeminence in the ghāziya, with the Ottoman sultans generally acknowledged as excelling all others in this feat:

For political reasons the Ottoman sultans attached the greatest importance to safeguarding and strengthening the reputation which they enjoyed as ghāzīs in the Muslim world. When they won victories in the ghazā in the Balkans they used to send accounts of them (sing., feth-nāme) as well as slaves and booty to eastern Muslim potentates. Knights captured by Bāyezīd I at his victory over the Crusaders at Nicopolis in 798/1396, and sent to Cairo, Baghdad and Tabriz were paraded through the streets, and occasioned great demonstrations in favour of the Ottomans. 
Cambridge History of Islam, p. 290 

Maghāzī Literature
Maghāzī, which literally means "campaigns", is typically used within Islamic literature to signify the military campaigns conducted by the Prophet Muhammed during the post-Hijra phase of his career. The record of these campaigns, usually conducted as traditional plundering raids, constitutes its own genre of prophetic biography within Islamic literature distinct from the sira. A famous example of the genre is the Maghāzī of al-Waqidi.


Operationally
When performed within the context of Islamic jihad warfare, the ghazw's function was to weaken the enemy's defenses in preparation for his eventual conquest and subjugation. Because the typical ghazw raiding party often did not have the size or strength to seize military or territorial objectives, this usually meant sudden attacks on weakly defended targets (e.g. villages) with the intent of terrorizing/demoralizing their inhabitants and destroying material which could support the enemy's military forces. Though rules of war in Islam's rules of warfare offered protection to non-combatants such as women, monastics, and peasants (in that, generally speaking, they could not be slain), their property could still be looted or destroyed, and they themselves could be abducted and enslaved (Cambridge History of Islam, p. 269):

The only way of avoiding the onslaughts of the ghāzīs was to become subjects of the Islamic state. Non-Muslims could then enjoy the status of dhimmīs, living under its protection. Most Christian sources confuse these two stages in the Ottoman conquests. The Ottomans, however, were careful to abide by these rules... Faced with the terrifying onslaught of the ghāzīs, the population living outside the confines of the empire, in the 'abode of war', often renounced the ineffective protection of Christian states, and sought refuge in subjection to the Ottoman empire. Peasants in open country in particular lost nothing by this change. 
Cambridge History of Islam, p. 285 
A good source on the conduct of the traditional ghazw raid are the medieval Islamic jurists, whose discussions as to which conduct is allowed and which is forbidden in the course of warfare reveal some of the practices of this institution. One such source is Averroes' Bidāyat al-Mujtahid wa-Nihāyat al-Muqtasid (translated in Peters, Jihad in Classical and Modern Islam: A Reader, Chapter 4).


Related Terms
akinji: raider, a later replacement for ghāzī 
al-'Awāsim: the Syrio-Anatolian frontier area between the Byzantine and various caliphal empires 
ribāt: fortified convent used by a militant religious order; most commonly used in North Africa 
thughūr: an advanced/frontier fortress 
uj: Turkish term for frontier; uj begi (march lord) was a title assumed by early Ottoman rulers; later replaced by serhadd (frontier) 

Contemporary Manifestations

Darfur
The conflict in Darfur provides perhaps a near-perfect recreation of the classical Islamic ghazw, despite the fact that many of its victims are themselves Muslims. In particular, the Janjaweed tactics of quick raids, destruction of property, and enslavement of non-combatants match the primary features of this institution exactly. Also in line with an aspect of the classical ghazw is the use of these raids to extend the domains of an Islamic state.


Islamic Terrorism
With its use to denote the July 7th London bombings in a statement claiming responsibility for the attacks, term ghazw became associated with contemporary Islamic terrorism.

The degree of resemblance between the two phenomena is a contentious issue. Among the key differences are Islamic terrorism's lack of discrete military function (i.e. territorial conquest) within the purview of a Muslim state, and its failure to obey certain traditional Islamic rules of warfare, including prohibitions against attacks on non-combatants. Yet these prohibitions were never understood as absolute by the classical jurists, who often conceived of them in conditional/prudential terms (see Averroes' Bidāyat). In any case attacks to demoralize/terrorize the enemy were allowed, and in its adoption of this function Islamic terrorism's resemblance to the traditional ghazw is greatest.


References
Encyclopedia of Islam, edition CD-ROM v. 1.0 (1999). Article: Ghazw 
Encyclopedia of Islam, edition CD-ROM v. 1.0 (1999). Article: Ghāzī 
Lewis, Bernard (1991). The Political Language of Islam, University of Chicago Press. ISBN 0226476936., p. 74 
Firestone, Reuven (1999). Jihad: The Origins of Holy War in Islam, Oxford University Press. ISBN 0195125800., p. 34 
Peters, Rudolph (1996). Jihad in Classical and Modern Islam: A Reader, Markus Wiener Publishers. ISBN 1558761098. 
Averroes, Bidāyat al-Mujtahid wa-Nihāyat al-Muqtasid 
Witek, Paul; & Heywood, Colin, translator (2002). The Rise of the Ottoman Empire, Curzon Press. ISBN 0700715002. 
Holt, Peter M., ed. (1970). The Cambridge History of Islam: Volume 1, The Central Islamic Lands, Cambridge University Press. ISBN 052107567X. 
Robinson, Chase (2002). Islamic Historiography, Cambridge University Press. ISBN 0521629365.