To those who are unknown about the Syrian Crisis.
Syria is a country bordering Lebanon and Mediterranean Sea to the west, Turkey to the north, Iraq to the east, and Jordan to the south and Israel to the south west. Bashar al-Assad has been the president of Syria since 2000 (elected in 2000, 2007 and 2014) and has been ruling till this date. Before that his father Hafez al-Assad was ruling as a President from 1971 to 2000.
Mass protest (Arab Spring/Revolution) in Syria began on January 2011 calling for the political reforms, re-instatement of the civil rights and end to the state of emergency which had been in place since 1963. During the protest in March the same year Syrian authorities responded with violence against its protesting citizens. In July the protestors started shooting back and were joined by some Syrian troops who called themselves the 'Free Syrian Army' (FSA). Then the uprising became a civil war. Then in 2012 a Kurdish group was formed that informally seceded from Assad's rule in the north and proclaimed them as 'Democratic Federation of Northern Syria'.
From 2012, Iran started to intervene on behalf of Assad. The Gulf States began to send money and weapons to the rebels (FSA) through Turkey to counter Iran's influence. In the same year, Hezbollah; a Lebanese military group backed by Iran invades to fight alongside Assad. Saudi Arabia responded it by sending more money and weapon to the rebels through Jordan.
In 2013, the Obama administration signed a secret order authorizing the CIA to train and equip Syrian rebels. In August, Assad regime used chemical weapons against civilians. Barack Obama then responded it saying that 'it is the national security interest of USA to respond to the Assad regime's use of chemical weapons through a targeted military strike'. After that the US arms reached Syrian rebels and USA became the participant of the war. Russia proposed Syria to surrender control over its chemical weapons to the International community for its eventual dismantling to avoid the US strike.
In 2014, al-Qaeda affiliate group based mostly in Iraq breaks away and ISIS (Islamic State of Iraq and Syria) was formed. Then the rebels were divided into two factions, one fighting against Assad and another against ISIS. The Kurds were also fighting against the ISIS. USA started bombing ISIS. But Turkey being allies of USA bombed Kurdish group and refrained it from bombing ISIS in August, 2015. In September 2015, Russia intervened on behalf of Assad saying they are there to bomb ISIS but they are alleged to be there against the rebels instead of ISIS.
Again in 2017, Assad used chemical weapon against its civilians. To respond it, USA launched dozens of missiles on an airbase in Syria. President Assad said the use of chemical weapons blamed to him was a 100% fabrication used to justify the U.S. strike. He also added that the 'Syria’s military had given up all its chemical weapons in 2013 after an agreement made at the time, and would not have used them anyway'. Putin defended him saying the use of chemical weapons was not done by Assad and was a provocation against Assad.
Till today around half million people (Civilians, militarist, rebels, etc) have lost their lives, around 7 million people have been internally displaced and more than 5 million people became refugee.
P.S. This is a very basic or surface situation of what Syrian Crisis is. States involved in the crisis has their own interest in Syria. It has become a playground for many international players and the Syrian crisis has remained not only as a Syrian civil war but also a proxy war where numbers of countries are playing. If you want to know more about it please go through as much sources as you can like news reported on BBC, CNN, Al Jazeera, etc, statements given by leaders, press release, etc. But make sure that you never rely on a single source as there is highly chance that it may be biased, so, be skeptic. And please correct me if I am wrong. 