What is YPG in syria?

Simple Answer
YPG is the Kurdish militant organization in northern Syria. They have had the greatest success both in repelling the Islamic State and the Assad Regime of any of the rebel groups in Syria. They have effectively created a functionally independent Kurdish State in northern Syria.

More Detail
The Kurds strewn between Turkey, Iraq, and Syria are not completely internally united and are actually made up of three main groups: the PKK paramilitary/terrorists in Turkey; the YPG, an offshoot of the PKK, in Syria; and the union between the PUK and PDK in Iraq, creating the KRG or Kurdish Regional Government. This division results in difficulty between coordinated attacks from Kurds in Iraq in Syria or in defense of the PKK in Turkey, which is under attack from Turkey. Given that the KRG uses Turkish military intelligence in targeting ISIL, the PKK has become suspicious that the PUK and the PDK know about impending Turkish attacks against their organization and are failing to provide advanced warning. YPG acknowledges that the KRG is stronger militarily than it is, but it does not want help from the KRG to translate into political or administrative power at a later point in time since the YPG is far more left-leaning economically than is the KRG. As a result, the YPG remains wary of accept military help from the KRG. The Kurds collectively maintain good relations with both Iraq and the FSA, since those groups have made provisions for Kurdish autonomy or independence. They jointly coordinate attacks on ISIL or Syria with these groups. The YPG is direct conflict with both Assad in Qamishli and ISIL further south, near Deir el-Zour, and further west, near Kobane. The Kurds collectively revile ISIL for its barbarity as well as its genocide against Yezidis (who are all ethnically Kurds) and Assyrians and other religious minorities (who the Kurds consider "their guests" in Kurdistan). The Kurds are the most successful Anti-ISIL group currently fighting.