there are multiple different sides in this conflict:
There are those on Assad’s side, the Baathists. Baathism incorporates some elements of Islamism but is ultimately a version of National Socialism which believes that Arabs, rather than Germanic groups (as believed by the original National Socialists) are superior.
There are several Islamic groups states such as Hezbollah and Iran which support Assad and belong to Shia Islam (more on this later in the post).
There are several Islamic groups which figure prominently in the rebellion such as Hamas and Al Qaeda which belong to Sunni Islam. (more on this later in the post).
There is the FSA (Free Syrian Army) which is the major rebellion group which is composed largely, though not totally, of Islamic terrorists.
There are several other rebellion groups such as those representing the Kurds which are generally at least partially pro western and are opposed to both the FSA and Assad.
The civil war began with protests grown out of the Arab Spring in March of 2011. This were peaceful protests which demanded reform and transparency which were cracked down on by Assad’s government. Eventually rebel groups began forming militias such as the FSA and began actively fighting back against Assad. By 2012 terrorists flooded Syria and began to play a prominent role in the uprising. The Arab Spring is a term for the group of protests which occurred in the Arab world. In December 2010 protests which resulted in the overthrow of the Tunisian government began in Tunisia, and spread the following month to Egypt. After a few months another revolution occurred in Yemen and a civil war began in Libya. These protests were originally supported by those supporting liberty, however many of them were organized by far leftest groups and the organizing of protests eventually fell into the hands of Islamic groups such as the Muslim Brotherhood.
Shia vs. Sunni
As I said before I would get to this. Generally Hamas and Hezbollah fight together against Israel, or at least support each other, however when conflict emerges that doesn’t involve Israel that partnership generally breaks down. They belong to two opposing sects of Islam, the Shia sect and the Sunni sect. The differences between the two revolve around the successor to Muhammad. The Sunni believe that a successor was to be chosen by an election of the Ummah (the Muslim community) to lead the Caliphate. The Shia believe in a more monarchist like system in which leadership stayed among Muhammad’s relatives. Sunni Islam is generally believed to be the more Orthodox sect of Islam, with Shia being akin to sort of a Protestantism of Islam. For more information this is a good overview
islam.about.com/cs/divisions/f/shia_sunni.htm