Part of the reason why the conflict in Syria is so complicated is because of the huge diversity of ethnic and religious groups living there. The rivalries and tensions between these peoples play an important role in the war and help explain what is going on.
The biggest ethnic/religious group in Syria are the Sunni Muslim Arabs, making up around 70% of the country’s population. Sunni Islam is most popular sect of Islam in the world, and is being supported and spread mostly by Saudi Arabian efforts.
The Sunni Arabs in Syria are the biggest supporters of the rebel forces, and in recent years have developed increasingly hostile feelings towards the Shia Muslim Assad government. They have always been the majority in a country that is lead by the minority. Feelings toward Shia Muslims have become so negative that chants such as “Death to Shiites!” and “Christians to Lebanon, Shiites to death!” have become quite regular in the Sunni Arab sphere.
Shia Muslims / Alawites
Shia Islam is the second big sect of after Sunni Islam, to which about 12% of Syrians adhere to. The majority of these followers are Alawites, as is President Assad. Although the Alawites are a minority in the country, they have held control over almost all aspects of the government since 1971 when the father of Bashar al-Assad took power. The Shiites around the world are mainly supported and funded by Iran, as a counterweight to Saudi Arabia. The Shiites are seen as heretics by many Sunni Islamists, and as such almost all Shias in Syria support the Assad regime, as they (somewhat justifiably) fear a massacre or even a genocide in revenge should the Sunni groups come to power.
The Kurds are a separate ethnic group in Syria and Iraq making up somewhere between 7-10% of Syria’s population. They have long sought autonomy, if not independence and the Syrian civil war provided an opportunity to finally make those dreams true. The Kurds stand out for being one of the few groups to fully recognize women’s rights, for being being secular (keeping religion seperate from the state) and for not discriminating based on religion or ethnicity. They are also one of the only groups who have not committed any unnecessary acts of violence in the course of the war. Thus, the Kurds are seen as the only real allies of the NATO countries in the war. It should be noted that there is one large exception to this, as Turkey, a NATO member, absolutely despises the Kurds and sees them as a grave enemy, having on occasions even bombed them. This of course creates a very tricky situation with the rest of NATO arming the Kurds and Turkey fighting them.
Most of the Kurds are also Sunni Muslims, but religion is not a determining factor for them.
The Syrian Turkmen are essentially ethnic Turks who have lived in the region since around the 11th century. They speak Turkish and are heavily funded and armed by Turkey itself. Although only making up around 1% of the population, they have been mentioned quite a lot in the news recently, as they were the group that captured and later killed one of the Russian pilots that were shot down by a Turkish F-16 fighter jet. The main groups have positioned themselves against Assad and ISIS.
The Assyrians are descendants of one of the oldest civilizations in the world, and see themselves as the original natives of the land they inhabit. They make up around 4% of the population. Assyrians are Christians and as such have been heavily discriminated against by extremist Islamist groups during the war. As a result, many Assyrians have taken up arms to defend themselves from the chaos. The armed groups are quite bafflingly named, as the pro-government one is known as Sootoro, while the pro-Opposition/Kurd one is known as Sutoro.
The Druze are followers of an 10th century off-shoot of Shia Islam. They make up around 3% of the population. Although they consider themselves Muslims, most of the Sunni Islamists do not agree. They are vehemently opposed to being conscripted to the Assad forces, often countering with weapons and sometimes even breaking Druze who refused to serve out of prison. As a minority group they too have been subjected to terrible abuse by some sides of this war. For example in the summer of 2015 Jabhat al-Nusra massacred a large amount of Druzes who refused to convert to their religion. In most cases, the policy of Jabhat al-Nusra (an al-Qaeda shootoff, read more here) is forced conversion and destruction of shrines. As for ISIS, their policy is violent annihilation of Druze without any mercy.