The most important thing for an outsider to understand is that Syria, while ethnically nearly 90% Arab, is made up of a rather large amount of religious minorities. Like most middle-east states, its borders were set up by Europeans during the colonial era, and really paid no heed to any actual cultural boundaries.
Only about half the population is Sunni Muslim, with most of the rest being Shi'a, and about 12% Christian.
Probably the most important religious group to take note of is the Alawites. This group is important because the country's current leadership is Alawite. This has a couple of important implications. First off, this particular minority is (perhaps rightly) afraid of what the rest will do to them if they lose power, and thus Alawites have not been very quick to join the uprising. There were efforts early on to recruit them into what has otherwise been a pan-Syrian uprising, but it doesn't seem to have borne much fruit. Some of the worst atrocities against the protesters have been carried out by "government-backed" militias, which I highly suspect are mostly Alawite.
The second important implication of this is that the current leadership, being Shi'a in a majority Sunni country, has traditionally had the benefit of strong support (really an alliance) from Shi'a-run Iran. If the current government falls, no matter what else happens, Iran loses. Fortunately, Syria doesn't share a border with Iran, or you'd likely see Iranian military units in the country "restoring peace". As it is, one can still expect Iran to do what they can to prop up their ally.
Another important player to keep an eye on is Hezbollah. This is a Shia (yes, often terrorist) militia in neighboring Lebanon. They are a big player in Lebanon, but are generally seen as a Syrian and Iranian puppet. They've more or less kept out of things so far, but they can't be happy at the thought of losing one of their patrons.
Another very important player here is Turkey. All of the middle east used to be ruled from Istanbul, and the Turks still like to think of themselves as the protector of their nearby Muslim neighbors. As a democracy (and Sunni) themselves, they are very sympathetic to what the uprising is trying to achieve, and they share almost all of Syria's northern border. So casus-belli incidents like Turkish planes getting shot down are very serious developments that the Syrian government should be trying very hard to avoid. Oh yeah, Turkey is also a full-fleged member of NATO, so if they engage in military action, they have the implicit (and likely explicit) backing of the USA.
Yet another important party is Russia. They have been sort of a minor ally to the current Syrian regime, and have a very important (to them) naval base agreement with them. They at one point took the step of sending in some marines to secure that base. If they wanted to intervene to support their ally, it would almost certainly start from that base.
