Q: Who are the YPG?
A: The YPG (Yekineyen Parastina Gel in Kurdish, or People's Protection Units in English) is a militia operating in majority-Kurdish areas in north and northeast Syria.
The group is the armed branch of Syria's powerful Kurdish Democratic Union Party (PYD).
The party was formed in 2004, after Kurdish demonstrations against the Syrian government, but became formally active in July 2012 when regime forces withdrew from Kurdish-majority areas.
The militia includes a female unit known as the YPJ, and is close to Turkey's Kurdistan Workers Party (PKK), which Ankara considers a terrorist organisation.
Q: Where has the YPG beaten IS?
A: Backed by US-led air strikes, the YPG expelled IS from the Kurdish border town of Kobane in northern Aleppo province in January after some four months of fighting.
It has since gradually chipped away at IS territory in the north of Raqa province, where the jihadists' de facto Syrian capital, Raqa, is located.
On June 16, the YPG and allies took Tal Abyad on the border with Turkey, depriving IS of a key conduit for transporting weapons and fighters and exporting oil.
The anti-IS forces have since seized a military base from IS and a town just 55 kilometres (35 miles) from Raqa city.
Q: Who fights in the YPG?
A: According to Kurdish affairs analyst Mutlu Civiroglu, many of its senior commanders are PKK veterans with extensive combat experience in Syria, Iraq and Turkey.
"This is one of the YPG's strengths, their commanders are well-respected and well-trained veteran fighters," he said.
A 2014 report from the International Crisis Group said the YPG paid salaries to between 25,000 and 30,000 fighters, but experts say there are no official statistics on its size.
Additionally, many Kurdish residents in areas targeted by IS have volunteered, swelling the force's ranks.
Q: How well-armed is the YPG?
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A: Civiroglu says weaponry is "the weakest point" for the YPG, which lacks a source for heavy weaponry and largely relies on old Russian arms and what it can capture.
During the battle for Kobane, some Kurdish peshmerga forces from Iraq were allowed to enter from Turkey with heavier weapons, but in limited quantities.
YPG commanders have regularly urged the West to provide them with more modern arms to fight IS, which is considered the region's best armed militant group.
Q: Why have they been successful?
A: Experts put YPG successes down to several factors, including the role of US-led air strikes.
"The US air strikes have been vital. They speed up the advances, they reduce the casualties and they eliminate IS's advanced weapons capabilities," said Civiroglu.
But experts note that the US-led coalition is equally dependent in Syria on the YPG to be its eyes and ears on the ground, and capitalise on its air strikes.
Wladimir van Wilgenburg, a Kurdish affairs analyst at the Jamestown Foundation, said the PKK's experience fighting conventional forces was also relevant to the YPG's battle against IS.
"The Islamic State is more and more acting as an army, rather than an insurgent group. So the YPG, which has been trained by the PKK, can fight them easily," he said.
"Other rebel groups, and even the Syrian government, do not have a similar experience, lack motivation, and coordination."
The YPG is also highly motivated because the Kurds see no possibility of coexistence with IS.
Syria's Kurds are Sunni Muslims but reject IS's harsh interpretation of Islam, espousing secular governance with female participation.
Q: Who backs the YPG?
A: The US-led coalition fighting IS has praised its cooperation with the YPG and allied rebels, but the militia is regarded by Turkey as an extension of the terror-listed PKK.
Experts say the YPG draws on funds including donations from Kurds in the diaspora and taxes collected in Kurdish-majority areas of Syria.