What is hapening in syria?

There is a civil war going on that is at the same time being used as a power struggle between various religious factions of Islam and the countries that support them. In simple terms, everybody is fighting everybody for a whole lot of different political and religious reasons.
It started with revolts against the clan of President Assad, who ruled with dictatorial power and who has his power base in the minority tribe (and religious faction) of the Alawites. The majority of people in Syria are non-Alawites who religiously belong to the Sunni form of Muslim believers.
These people are usually painted as the 'moderate' rebels, although they are certainly not out to establish a western style, multi-party democratic system. The Syrian, Sunni rebels are only moderate in that they don't impose a reign of terror over the territories they occupy.
The Alawite clan of president Assad has friendly relations with other, Shiite Muslim groups. Iran is the major Shiite power in the region, so it actively supports Assad partly directly, partly through a "freedom fighter" organization it supports called Hezbollah. Russia also actively supports Assad, a) because his regime always had friendly relations with Russia and b) they don't expect a future Sunni leadership to be as friendly to them.
The Sunni Muslim rebels get support from othe Sunni Muslim nations like Turkey, Saudi Arabia and other countries on the Arabian peninsula. And from the West which apparently and rather optimistically thinks that the rebels will insitute (after winning, of course) a multi-party, democratic and stable government that will be friendly to the West - in contrast to the largely pro-Russian present Assad government.
But most of all, the Sunni uprising against Assad brought in the also Sunni battle groups of IS. The problem with them is that they do institute a reign of terror in the regions they occupy they are totally anti-West and they are there not to help the 'moderate', Syrian Sunni brothers. Although they also want the downfall of Assad they want Syria for themselves to set up their own Islamic Empire.

So now the West is helping some of the rebels against Assad, the Syrian ones, but actively bombing other rebels, namely IS. It has no problem with the fall of pro-Russian dictator Assad, but on the other hand much fears the chaos that might ensue after his fall. Russia helps Assad and bombs mostly his Syrian rebels because it does not want a hostile Sunni power bloc along its southern borders in the future - and it sees that the West and others are already fighting IS. Iraq and Saudi Arabia support the uprising of their Syrian fellow Sunni Muslims but at the same time (half-heartedly) fight the also Sunni IS. And Saudi Arabia and Iran are basically using Syria as a battlefield in their own fight for supremacy in the region.
At the same time, the Kurds are fighting against IS which gets them training, arms and support from the West. But they are also fighting for autonomy or who knows, independence which is a reason that Turkey (also a supposedly Western ally) fight the Kurds.
This makes it understandable that in a situation like this President Obama very wisely is not committing any US ground troops there. Because before he knows what happens, they will be shot at from all sides: nothing unites Muslim warrior groups like the chance to use Western unbelievers for target and bombing practice.