As an addendum to my prior post, here's a brief description of the fighting forces mentioned within the article:
YPG/YPJ: The YPG are the Kurdish self-defense forces native to northern Syria in the de facto Kurdish autonomous region currently known as Rojava. Almost exclusively light infantry, the YPG are known for leveraging martial discipline and tactical savvy to compensate for limited weaponry. Initially modeled after the Kurdish PKK in Turkey, the YPG are radically egalitarian and have a female fighter's component known as the YPJ. Kurdish women fighters serve as commanders across numerous fronts in Syria; the gender equality seen in the YPG and among Syrian Kurds in general is held as a possible catalyst for social change among the Kurds as a whole.
Peshmerga: Literally "one who faces death", the Peshmerga are the national army of Iraqi Kurdistan. Possess a range of capabilities including heavy armor and spec ops in addition to light infantry. Are also held as well-trained and skilled, but have received much more explicit support and funding from the United States than the Syrian Kurds. Though more conservative than the Syrian Kurds, Iraqi Kurdish women have some presence in the Peshmemrga. Like the YPG/YPJ, are exclusively staging operations against ISIS at the moment.
Hezbollah: The armed wing of the militant political organization also known as Hezbollah ("Party of God"). Conservative Shia hardliners with deep connections to Iran who are explicitly hostile to the United States and Israel. Headquartered ostensibly in Lebanon, serve as allies to Iran and their Shia partners - Syria's Bashar al-Assad in particular. Are known for being low profile and keeping their operations quiet; the exact size of their military corps isn't entirely clear. Fought Israel during the 2006 Lebanon War, effectively to a draw. Also execute terror and sabotage operations, and are known as talented fighters.
ISIS: You know who these assholes are.
al-Nusra Front Explicitly Sunni sectarian jihadi militia currently fighting and holding terrain in the Syrian civil war. Serves as the Syrian branch of al-Qaeda. Formed in early 2012, had tenuous alliances with ISIS before descending into violent antagonism with the group in 2013. Have been openly fighting with ISIS ever since. Many al-Nusra fighters consider ISIS to have betrayed the tenants of hardline Salafist Islam and deviated from the dictates of God. There's been analyst chatter that al-Nusra may have recently split from al-Qaeda and declared organizational autonomy. Currently fighting for territory in Syria's metropolitan west, a position that puts them up against both the FSA and Bashar al-Assad loyalists.
Free Syrian Army (FSA): Primarily defectors from Bashar al-Assad's Syrian army, mixed with local rebels and enlisters. More of a general rebel faction than a hardline jihadist faction like ISIS or al-Nusra, though some FSA fighters have defected to jihadi groups. Are most interested in fighting and deposing Bashar al-Assad, and have been the primary targets of Russia's recent airstrikes. Have proven surprisingly resilient and successful against Assad's forces, and have made momentary alliances with the Syrian Kurds against ISIS.
Syrian Arab Army (SAA): The majority-Shia fighting forces still loyal to Bashar al-Assad. Most of the Sunni officers defected to the FSA, and the SAA has become more closely aligned with the Iranian axis after the Syrian civil war broke out. Considered a subpar fighting force since 2013, have received heavy reinforcement from Iran and Hezbollah that has managed to counterbalance their strategic deficits. There've been mounting questions as to how long the SAA will be able to hold out. Certain analysts have suggested the entry of Russia into the Syrian conflict was spurred by the questionable longevity and competence of the SAA, similar to the recent upsurge in support from Iran/Hezbollah.
Header photo of a Kurdish YPJ regiment. Photo from Flickr Creative Commons.