Who is fighting who in Syria, and why are they fighting? Can anyone give me a very simple summary?

Assad's government is socialist and they locked up Fundamentalist Muslims so different factions of Muslims went to war on the legitimate government. ISIS thought they would expand into Syria from their stronghold in Iraq and fill a vacuum that had been created by the war. The western governments wanted to attack the Syrian government as some deal with the Muslims but changed their minds. Then ISIS blew up a Russian airliner over Egypt so Russia went to war in Syria to help the government by attacking ISIS and the west is now using aircraft to attack ISIS in Syria.

Right.

The Syrain Government are fighting the Rebels and ISIS, but is saying they are all ISIS.

The Rebels are fighting the government, but losing to ISIS because they didn;t realise ISIS were stealing their members.

ISIS are fighting both the Rebels and the Government.

Russia is helping the Syrian government to fight both the Rebels and ISIS.

France are fighting ISIS, and occasionally hitting the Rebels by accident.

Everyone else who is involved (UK, US, etc) are trying very hard to just hit ISIS, but are also supplying arms to the Rebels to fight the Government, some of which ar ebeing stolen and taken to ISIS.

And on at least one occasion, the US were attacking a civilian charity hospital run by Medicins Sans Frontieres (Doctors Without Borders).

i think it is isis versus assads forces.

Get lost. Go read the news.