The Turks were a Central Asian people of various tribal groupings, apparently originating in the Altai Mountains. They moved into Transoxiana in the fourth to sixth centuries AD, raiding the eastern Sasanian frontiers. They began to cross the Oxus from Central Asia and move westwards into Persia’s interior in the eleventh century, led by the Seljuk (the anglicized form of Saljuq) of the Oghuz or Ghozz tribes, a movement which eventually led to the creation of the Ottoman empire and modern Turkey, First they encountered the Ghaznavids, whom they chased out of Khorasan; then the gateway was open for conquering the Iranian hinterlands, Iraq, Syria and Anatolia...
As mentioned earlier, the Turks had replaced most of the troublesome nomads on the eastern reaches of the empire. The early Abbasids, recognizing the Turks’ martial qualities, had used individual Turks as military (and to lesser extent domestic) slaves. Within a short period these began to dominate the Caliph and interfere with the affairs of the caliphate. Likewise, such eastern Persian states as the Samanids had later taken Turkish slaves into their (mainly military) employment. This too had in time led to the rise of individual Turks, such as Alptegine and Saboktegin, who founded the Ghaznavi empire. Yet none of this had been due to invasion or the mass migration of Turkish hordes into Persia and beyond. The privilege first went to Seljuk Turks, who prepared the ground for later migration and invasions. Thus it was the steady and continuous of Turkish nomads for the centuries following the Seljuks that resulted in the considerable linguistic and ethnic impact of the Turkish people on Iranian civilization.