7. There have been efforts to unite the rebels, but still there's no central figure leading them. And if the regime falls, it's anybody's guess what could happen next.
An organization known as the Supreme Military Council, which formed late last year, now unites many rebel groups.
And for now, the rebels are working together to achieve a common goal -- toppling al-Assad's government. Here's how one local al-Nusra front leader put it to CNN in April: "In the period after the regime falls, our main goal is to create an Islamic state that is ruled by the Koran. It can have civilian institutions, but not democracy. We look at the other Free Syrian Army rebels as one of many groups defending religion, so we support them. In the future, we will handle this differently."
8. Religion motivates many of them.
The rebels are largely made up of Sunni Muslims battling against al-Assad's minority Alawite sect, which is associated with Shia Islam. Weapons and funds from Iran's Shia rulers have helped the Syrian regime, while Sunni states like Saudi Arabia have reportedly supported Syrian rebels.
"The conflict has become increasingly sectarian, with the conduct of the parties becoming significantly more radicalized and militarized," the UN said earlier this year.
That doesn't bode well for Syria's future. Studies have said religious civil wars are longer and bloodier than other types of clashes. They're also twice as likely to recur and twice as deadly to noncombatants.
9. They're not all Syrian.
Thousands of foreign fighters are believed to have traveled to Syria to join the rebels since early 2011. The Washington Institute for Near East Policy estimates the total at between 2,000 and 5,500.
That group includes hundreds of Europeans, the institute says. And there have also been reports of several people from the United States fighting with the rebels.
10. They're not just fighting al-Assad's regime -- they're fighting Hezbollah militants.
Early on in the Syrian conflict, reports surfaced that Hezbollah fighters were helping Syrian government forces. In May, the Lebanon-based Shiite militant group's leader confirmed it.
"Syria is the backbone of the resistance (in the region) and its main supporter," Hezbollah Secretary-General Hassan Nasrallah said in a televised speech. "The resistance will never stand by while its backbone is exposed."
11. It's not all about fighting on the battlefield.
The Syrian National Coalition is an umbrella group representing the Syrian opposition that formed last year.
They've met repeatedly with regional and Western political leaders and diplomats, pushing for financial and military assistance with al-Assad's ouster.
12. The rebels have gained significant ground in some parts of the country, but militarily, they're outmatched.
While rebels have won territory in key areas, like northern Syria, they've had trouble purging out pockets of regime strongholds. The Syrian military's air power leaves them vulnerable. And the Syrian government's grip on many areas of the country is tight.
13. The rebels get weapons from a variety of sources, including foreign governments.
That's shifted the balance within the rebels and strengthened more moderate groups among them, according to Elizabeth O'Bagy with the Institute for the Study of War.
"Saudi Arabia and a number of allied countries actually began to empower the more moderate forces through a train-and-assist program in which they were providing weapons and providing assistance," O'Bagy told CNN Thursday. "That had a significant impact on empowering these groups and giving them the capacity to marginalize the extremists and assert their own authority."
Sympathetic Sunni groups from other countries have also helped arm the rebels. And rebels have raided regime weapons stockpiles for supplies.
14. There are between 70,000 and 100,000 rebel fighters.
That's the estimate U.S. Secretary of State John Kerry gave during congressional testimony this week.
15. How many of them are extremists? It depends on who you ask.