http://www.bbc.com/news/world-middle-east-29532291

Right now the Popular Protection Units (YPG) fighters are engaging in
fierce street battles with IS fighters.

The YPG is the armed wing of the PYD fighting for Kurdish independence
in Syria.

https://en.wikipedia.org/wiki/Democratic_Union_Party_%28Syria%29
https://en.wikipedia.org/wiki/People%27s_Protection_Units

They are members with the other Kurdish organisations under the umbrella
of the KCK, and is ideologically aligned.

"The Democratic Union Party considers jailed PKK founder Öcalan as its
ideological leader, and declares the People's Congress of Kurdistan
(Kongra-Gel) as the supreme legislative authority of the Kurdish people.
It incorporates into the United Kurdish Community in Western Kurdistan
(KCK – Rojava) with its own organisational identity."

"The PYD admits that the two parties have an ideological affinity and a
close relationship"

The ideology of the KCK:

"The democratic confederalism of Kurdistan is not a State system, it is
the democratic system of a people without a State... It takes its power
from the people and adopts to reach self sufficiency in every field
including economy."

The YPG is not only representing Kurdish interests, as they have been
fighting Islamic fascists since the beginning of the Syrian civil war:

"Though predominantly Kurdish, the group has attracted increasing
numbers of Arabs, including fighters defecting from the mainstream
opposition[13] as well as locals from mixed or Arab villages in
YPG-controlled territory who see the group as the best guarantor of
regional security"

The war is polarising between a diverse mixed group of peoples led by
the Kurds linked to anarchist philosophy, and Sunni tribes under the
banner of the IS.