This is really just a dump of what I've been learning about what's been going over here in Thailand. There are long term themes (going back to the elections that first brought Thaksin in and even to long before that) and short term themes (the current crack down and breaking up of the red protests in central Bangkok and the resulting chaos).
As always with Thailand there are wheels inside wheels here.
The simple way of looking at this is that the red shirts are protesting to get Thaksin back into power after he was ousted in the coup in 2006. The red shirts are largely rural poor who say that they've had their lives improved immeasurably under Thaksin and want him back.
The yellow shirts on the other hand point to Thaksin's corruption, censorship and obvious use of foreign policy to further his own business ends.
The exact point scoring between the two sides quickly descends into madness and eventually trivia. Of course nobody really knows what either side wants to say as every thing is censored and the only people who get to talk to anyone are the army through their media assets.
Behind that is the current government which came in “democratically” after the party that won the election was banned by the courts (it's illegal to comment on that decision so I won't). This government is supposedly supported by the army who have really been calling the shots since the coup (actually, all the time except for maybe under Thaksin).
Now, the reds are not army supporters (and that's from even before the army started shooting at them over the last few days) — or at least, they're not so in the same was as the yellows — so who controls the government in September is critical. This is because that the current army head (and some would say the actual power in Thailand) retires then and the government gets to choose who is next. Clearly the yellows would like one of their people in there. Notice Abhisit (the Prime Minister) offered elections in November — enough time for the new head of army to get his feet properly under the desk before the reds win the next election.
Behind this of course is a fight for the succession, but Thai laws certainly don't allow discussion about any of that.
Some bonus points:
There is so much more, but I don't really have the patience right now to try to go through it all.