Pottery from Iran (about 1100 AD)
The Seljuks were Turkic nomads from Turkmenistan, related to the Uighurs, who entered the Abbasid empire around 950 AD and gradually converted to Sunni Islam. By 1030 they were beginning to try to get power for themselves, and they soon conquered the Ghaznavids (wwho were also Turkic) and controlled most of Persia (modern Iran). Their capital was at Isfahan. Like the Ghaznavids, the Seljuks spoke Persian and encouraged Persian culture.
By 1055, the Seljuk king Togrul Beg had conquered Iraq too, and although there was still an Abbasid caliph in Baghdad, the caliph pretty much had to do whatever the Seljuk sultan Togrul Beg wanted him to do. When Togrul Beg died in 1063, he was succeeded by his nephew Alp Arslan (Alp Arslan means "the hero lion" in Turkish, and it's where C.S. Lewis got the name of Aslan for the Narnia books).
In the west, Alp Arslan fought to take Syria from the Fatimids and Armenia from the Byzantine empire. In 1070, the Byzantine emperor Romanus IV Diogenes decided to try to push the Seljuks out of Armenia. At first
the Byzantines (with some Norman allies) won. Then there was a great battle at Manzikert in 1071, just east of the Euphrates river, but the Byzantines lost. The Seljuks not only won the battle, they also captured the Byzantine emperor Diogenes!
In the end, Alp Arslan let Diogenes go in exchange for a lot of gold and the promise that he could have Armenia and a lot of other Byzantine land too. Then they were at peace.
Alp Arslan died the next year and his son Malik Shah became sultan. But after Malik Shah died in 1092, the Seljuks got less powerful, and by 1192 their dynasty ended. The Seljuk empire broke up into many small kingdoms.
A Seljuk fort near Istanbul at Alanya, built in 1226 AD
When the Mongol invasions came to West Asia, the Byzantines and the Seljuks fought together against the Mongols. But they lost, and in 1243 the Mongols took over both Iran and Anatolia (modern Turkey).
After the Mongol Empire collapsed, however, it was one of these Seljuk kings, Osman, who founded the new Ottoman Empire.