It's not an easy question to answer. In the beginning of the conflict, swaths of anti-Assad protestors banded together to form the Syrian National Council. The group was centralized out of neighboring Turkey. Their numbers grew and by 2012, additional rebel groups joined the Syrian National Council and changed their name to the Syrian National Coaltion. The group claimed it was the legitimate Syrian government body and even managed to gain recognition from neighboring Gulf states as well as Western countries.
At the same time, there was a mass exodus of troops from the official Syrian Army. These were people who wanted to defect from the Assad regime, forming the Free Syrian Army. The FSA was initially made up mostly of Sunni Muslims. This contrasts with Assad's forces, that are mostly Alawite Shia Muslims. In 2011, there was a massive release of Salafist prisoners in Syria. Rebel groups allege this was a deliberate tact by Assad to breed extremism among rebel groups. Indeed many of these prisoners went on to join extremist groups, some even became leaders within ISIS.
Analysts estimate there are over 1,000 loosely connected rebel groups currently fighting in Syria. Some are directly fighting Assad's army, while others are fighting ISIS fighters. On top of that, some rebel groups are fighting Kurdish militias, an ethnic group that is pushing for independence. It's a very complex situation, made even more complex by the involvement of major armed forces like Russia, Iran, Saudi Arabia, and the U.S.
What's happening in Syria? (bbc.co.uk)
"The violence in Syria began in March 2011 and the middle eastern country has been crippled by a brutal civil war ever since."
U.S. Congress approves arming Syrian rebels, funding government (reuters.com)
"U.S. Congress approves arming Syrian rebels, funding government"
Putin's military intervention in Syria, explained (vox.com)
"Russia has moved a small but significant military force into Syria, adding a volatile new dimension to Syria's now four-year civil war."
Syria: who are the rebel groups fighting against Assad? (theweek.co.uk)
"Islamic State militants and Syrian government forces are said to be encircling the key city of Aleppo, in what some believe could be a tipping point in the three-year civil war."