This is a really complex question and has many sides. I will offer some of my own knowledge:
The roots of the Syrian civil war are linked to the Arab Spring and to older trends in Syria. Of the latter, the country, mostly Sunni, is ruled by a small Shiite elite. Highly corrupt, the Assad regime was a typical target during the Arab Spring.
The Syrian army, rather massive, and largely of Russian equipment, is loyal to the Assad regime. It's role in the war is more than twofold. Of the obvious ones are that it gives Assad hard power (although they nearly lost the war before Russian intervention) and that it represents one of the several interests Russia has in this war. Syria has access to the Mediterranean and is bordered with Iraq, Joran, Israel, Lebanon and Turkey.
What's the war about? Well for Assad it was about remaining in power initially. Now it is about focusing on returning to his full control mode without giving up too much to the Iranians in Syria. On the opposite side, there are many factions and each look for either sovereignty (Kurds), or something more complex.
For Iran, the most obvious interest is a land-corridor stretching from heavily Iranian influenced Iraq to Lebanon, where Hizbollah is a major player. This type of land reach allows Iran to directly supply its proxies with weapons and have access to the Mediterranean. The proxies play a key part of Iranian strategy in that they further its conflicts from its homeland, and buys them influence.
For Russia, it seems the most obvious interest is a veneer of influence and power. Russia plays a role of mediator between the Assad regime, Iran and its militias, Israel, and other smaller factions. Russia also has interests in Syria in oil production and weapon sales.
For Israel the Syrian civil war is turning to be about preventing Iranian military establishments in Syria. Having them would mean a much more convenient passage to Hizbollah in Lebanon. Moreover, the Israelis want to avoid a double front in its northern border. Israeli policy of the war doesn't seem to be about trying to influence the fate of the regime, or its successor, but more to halt Iran.
The U.S interests in Syria are to me more puzzling. Most obvious are containing Russia and Iran, and securing Israel. The Americans have alliances in the region with Egypt, the Saudis and Israel, among others. It is not clear to me how might the U.S administration react to Iranian entrenchment in Syria. I guess I should mention that IS played a major role in attracting the U.S into this conflict, but I can't say that they show intent on managing it currently.
Hopes this gets you started,