Ahrar al-Sham Harakat Ahrar ash-Sham al-Islamiyya (Arabic: حركة أحرار الشام الإسلامية Ahrār ash-Shām, meaning "Islamic Movement of the Free Men of the Levant") A coalition of Islamist brigades that "cooperates with the Free Syrian Army and other secular rebel groups; however, it does not maintain ties with the Syrian National Council," and have "their own strict and secretive leadership, receiving the majority of their funding and support from donors in Kuwait." Ahrar al-Sham was a founding and leading member of the Syrian Islamic Front, and was a member of the Turkish-sponsored 2015 Idlib "army of conquest" along with Al-Nusra. It's active mostly in north-western Syria.
Despite its hardcore Islamist ideology and personnel overlapping with Al-Qaeda, the group is endorsed by former US ambassador to Syria Robert Ford and other voices inside NATO, especially Turkey which refuses to even pro forma brand the group as terrorist in nature.
Links to Al Qaeda and ISIS
A prominent member of Ahrar Al-Sham, Mohammed al-Bahaiah (aka Abu Khaled Al-Suri, their leader for Aleppo) was a longtime associate of Al Qaeda's leadership, but Ahrar Al-Sham contends he was never a formal Al-Qaeda member. 
Ahrar Al-Sham was a founding group, alongside with Jabhat al-Nusra (Al Qaeda's official branch in Syria) in the 2015-16 Idlib Army of Conquest. The group has collaborated with Al-Nusra in various massacres including Latakia (August, 2013), Ishtabraq (April, 2015) and Zara (May, 2016) A former Nusra "All I know is that Nusra sees Ahrar as their source for weapons, especially in some battles," he said, noting that currently they were doing less fighting themselves and focusing on administration (leaving the fighting more to a well-armed JaN?) 
Ahrar Al-Sham also had earlier links to Islamic State (ISIS, ISIL, Daesh), before it was excommunicated from the "moderate" Islamist circle. In May 2013, Al-Qaeda selected their associate, Ahrar Al-Sham member Mohammed al-Bahaiah, to mediate between Jabhat al-Nusra and the then-emergent ISIS, as they forged their initial alliance.  ISIS was then one of the five core groups, alongside JaN and Ahrar Al-Sham, in the August, 2013 Latakia Massacres (see below).
Their October, 2013 statement denying any wrongdoing in that was from "Ahrar al-Sham's political office in Raqqa."  Raqqa was taken over by Islamists in March, 2013, and then taken by Daesh (ISIS) as their capitol in mid-August, just as this Latakia offensive was ending. So if Ahrar al-Sham had its offices there two months later, it suggests they were working with the Islamic State at that time.
Ahrar al-Sham is one of five core groups clearly implicated by a detailed Human Rights Watch investigation in the premeditated and blatant Latakia Massacres of over 200 Alawi civilians on August 4, 2013 - (the others are ISIS, Jabhat al-Nusra, Jaish al-Muhajireen wal-Ansar, and Suqour al-Izz). HRW found they admitted to involvement in four villages," (Isterbeh, Nabata(misspelled), Hamboushia, Abu Makka) and posted video of them breaking into homes in a fifth (Balouta). Three Moroccan fighters from Ahrar al-Sham were listed as killed in Esterbeh on August 4." 
The group initially held no hostages but in September was given responsibility for them all. An "opposition activist" told HRW “Abu Taha protected Alawite women from the foreigners [other fighters] who wanted to kill them.” At least six naked dead women were evidenced in Hamboushiya, as the report notes. 
On the day that report was released, October 11, 2013, Reuters reported a sort of denial from the group: "If someone uses a weapon against you, you have to fight them. If they do not, you must not kill them," and that's it. The unstated half could be that the Alawites raised the sword against them (all Sunnis) so the Alawites have to die, but ostensibly they claim to have only killed fighters. 
HRW reports "In one of the videos, lieutenant colonel Hussein al-Harmoush, the brigade commander from Harakat Ahrar al-Sham al-Maghdad al-Aswad battalion, is identified by name and seen shooting in the operation."  That's either a nickname taken in honor of an FSA Godfather, the late Lt. Col. Hussein Harmoush (external profile) or quite a coincidence.
Other Massacres with alleged involvement:
- Nafeez Ahmed, Ahrar al-Sham’s apocalyptic vision for Syria and beyond, Middle East Eye, 16 October 2015
- Gareth Porter Why the Obama Administration Is Favoring al-Qaeda's Main Syrian Ally Truthout, June 10, 2016
- Charles Lister Syrian Islamists reach out to the U.S., but serious issues remain Brookings, Markaz blog, July 15, 2015
- Syrian rebel leader was bin Laden’s courier, now Zawahiri’s representative, Long War Journal, December 17, 2013
- Syrian Rebel Leader Subhi al-Refai on U.S. relations with Ahrar al-Sham, Aron Lund, August 12, 2015
- Syrian Islamists reach out to the U.S., but serious issues remain Charles Lister, Brookings, Markaz blog, July 15, 2015
- Resilient insurgent group Ahrar al-Sham to play bigger role in Syria By Mariam Karouny, Reuters, September 22, 2015.
- Reuters, Oweis October 11, 2013
- "You can still see their blood" - Executions, indiscriminate shooting, and hostage taking by opposition forces in Latakia countryside. PDF, 113 pages. Researched and written by Lama Fakih, Human Rights Watch, October 11, 2013. Read or download page
- Reuters, Oweis October 11, 2013