In 2001, a man named Thaksin (pronounced with a hard T as in Thai) was named prime minister by his majority party in parliament. He is from Chiang Mai, in the northern part of the country and had strong support from the north and northeast because he changed Thai financial policies to spread more of the wealth to the rural areas he came from which previously had been going mostly to Bangkok. His followers are the Red Shirts.
In 2006 Thaksin was tossed out in a bloodless coup, and fled the country rather than defend charges of corruption - he had sold his cell phone business to a company which was (a) outside of Thailand and (b) owned by his wife.
Two years later the military junta held democratic elections, and through massive voter fraud Thaksin's party won in a landslide and named him PM again. When he tried to return to Bangkok, 20,000 protesters occupied the airport, shutting it down for weeks. Those are the Yellow Shirts. Among the charges against him is lèse majesté and yellow was chosen because it is the King's color. The protest disbanded when the national court convicted Thasin's party of fraud, threw them all out of Parliament, and banned them all from politics for 5 years. The protesters not only left peacefully, they cleaned up the airport better than before they had been there. I was in Thailand at the time, and the protest forced me to fly home from Singapore instead of Bangkok as scheduled.
This March, with funding from Thaksin, who apparently is in exile in Dubai, red shirts from the north and northeast took over key intersections in Bangkok, as well as setting up a stage and major protest camp at Bangkok's equivalent of Hyde Park. They also blockaded Parliament. Many were heavily armed. In May a police swat team broke up the main blockade and camp area and arrested the leaders, but red shirts in other parts of the city and a couple of up-country sites went on a rampage. They burned down the central shopping mall, vandalized other businesses and burned down one or two community centers up-country. 88 people were killed.
And that's it in a nutshell. You don't even want to open the nut to see what's inside. :-)