YPG
The People's Defense Units (YPG) is an armed Kurdish group located in Syria and a combatant in the Syrian Civil War. Founded in 2011, the group has seen rapid growth over the years and makes up the major fighting contingent of the US-backed Syrian Democratic Forces (SDF). However, the YPG has also been designated a terrorist organization by Turkey and Qatar due to its association with the Kurdistan Workers' Party (PKK).
What will Turkey do with Syria's Kurdish population? - opinion
Although Erdogan might deplore the effect on Turkey’s domestic political scene, he may yet see an autonomous Kurdish region recognized within a new Syrian constitution.
Turkey moves to isolate the Kurds in Syria, and prepares a way for new attacks - analysis
The main group in Ankara's cross-hairs is the Syrian Democratic Forces, or SDF, which was formed in 2015 with US backing.