The People's Alliance for Democracy has shut down the Bangkok airport in order to overthrow the current government of Thailand. MSNBC has a video report.
If you don't want to sit through the advertisement and then the excerpt from NBC news, what is happening is that a political group of mostly middle class Thai people are protesting against the current democratically elected government of Thailand. The problem? The current government was elected by the relatively poor rural Thai citizens of northeast and northern Thailand.
The protesters are attempting to get the army to step in and conduct another coup, much as they did in 2006, and replace the current government with one elected after enacting voting limitations that disenfranchise the rural voters who support this government.
For an interesting and more in-depth discussion of the current Thai political situation, read Thailand's political maze – a beginners guide. Don't overlook the comments, a number of which appear to come from Thai individuals who have knowledge of the situation.