These are some of the main ones:
HAYAT TAHRIR AL-SHAM
The most powerful group in Syria that spearheaded the rebels' advance is the Islamist group Hayat Tahrir al-Sham.
It started out as the official al Qaeda affiliate in Syria under the name Nusra Front, staging attacks in Damascus from early in the uprising against Assad.
Its leader Ahmed al-Sharaa, who for years used the nom de guerre Abu Mohammed al-Golani, decided to split first from the nascent Islamic State group, and then in 2016 from the global al Qaeda organisation.
It went through several name changes, eventually rebranding as HTS, as it became the strongest group in the main rebel enclave around Idlib province in the northwest.
HTS and its leader have been designated terrorists by the United States, Turkey and others, but continued to fight alongside mainstream rebel groups and backed an administration in Idlib that they called the Salvation Government.
Sharaa has presented a more moderate image during the lightning campaign that brought down Assad, but some Syrians will probably remain fearful about his ultimate intentions.
OTHER REBEL GROUPS
Syria's uprising was deeply fractured, with a confusing mosaic of local groups espousing a range of Islamist and nationalist ideologies.
Over the years some of these splintered further or merged with other groups.
Coalitions, such as the Free Syrian Army and the Islamic Front, held influence at different periods of the conflict.
Their relative power was also shaped by whether they were based in regions captured by Assad or remained out of his hands.
In northwestern Idlib, which until last week's stunning advance was the main rebel stronghold in Syria, a range of groups fought alongside HTS in a unified military operations command.
Other groups had dominated in the south. A string of Assad victories in 2018 forced them to accept his rule but without turning over all their arms or coming back under full Damascus control. Last week they rose up again, taking southwestern Syria.
SYRIAN NATIONAL ARMY
Turkey sent troops into Syria from 2016 to push Kurdish groups and Islamic State away from its borders.
A key supporter of the rebels, it eventually formed some of the groups into the Syrian National Army which, backed by direct Turkish military power, held a stretch of territory along the Syrian-Turkish border.
As HTS and allied groups from the northwest advanced on Assad last week, the SNA also joined them, fighting government forces and Kurdish-led forces in the northeast.
SYRIAN DEMOCRATIC FORCES
The Kurdish-led Peoples Protection Units (YPG) took control of large areas of northeast Syria in 2012 as government forces pulled out to fight rebels in the west.
Turkey sees the YPG as inseparable from the Kurdistan Workers Party (PKK), which has waged a decades-long insurgency inside Turkey, and which the U.S. regards as a terrorist group.
As Islamic State advanced in Syria in 2014, the YPG joined other groups to hold them back, supported by the U.S.
They formed the Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias, backed by the U.S. and its allies.
The SDF now controls most of the quarter of Syria that lies east of the Euphrates, including the former Islamic State capital of Raqqa and some of the country's biggest oil fields, as well as some territory to the west of the river.
Its forces have been fighting the Turkey-backed SNA around the city of Manbij.
