The red-shirts are formally known as the United Front for Democracy Against Dictatorship (UDD). The focus of many red-shirts' campaigning zeal is former Prime Minister Thaksin Shinawatra.
Members are mainly rural workers from outside Bangkok. But the red-shirt ranks also include students, left-wing activists and some business people who see attempts by the urban and military elite to control Thai politics as a threat to democracy.
The source of the red-shirts' discontent goes back several years. They began as supporters of Mr Thaksin, the prime minister ousted by the military in a September 2006 coup.
Image caption The red-shirts are mainly rural workers from outside Bangkok
By then Mr Thaksin - a telecommunications magnate - had governed Thailand for five years. He was very popular among the rural farmers and urban working class because he initiated policies that benefited them, such as funding for health-care and education.
When elections were held 18 months after the military coup, this rural support had not changed, even though Mr Thaksin was in overseas exile.
Voters from Thailand's north and north-east returned his allies to power, only to see the government fall after a series of opposition protests and court rulings. So the red-shirts began protesting.
Their first major protest began in March 2009 with a series of sit-ins outside government offices, but quickly escalated.
In April 2009 they forced the cancellation of a regional political summit after storming the venue in the seaside resort of Pattaya.
Violence then erupted in Bangkok. Clashes involving troops, protesters and Bangkok residents left at least two people dead and dozens hurt.
As troops massed, the red-shirts called off their protests. Leaders said they feared more loss of life.
But their anger had not gone away and, in March 2010, they called fresh protests in Bangkok aimed at toppling the government.
Tens of thousands of people occupied Bangkok's historic and commercial districts and at one point stormed parliament, forcing MPs to flee. Red-shirts also stormed a satellite transmission base, in a bid to restart a television station which had been shut down by the government.
The first bloodshed occurred on 10 April when at least four soldiers and 17 civilians were killed in clashes as the army tried to disperse the red-shirts from one of their two bases in Bangkok.
The violence shocked the city - but the red-shirts consolidated their forces in one camp, closing down the city's commercial heart for several more weeks.
On 19 May armed government troops moved into the red-shirt camp, smashing through barricades. By the end of the day, the camp had been cleared, several of the group's leaders arrested and dozens of people, including protesters and soldiers, killed.
A year on, many of their leaders have been released on bail.
The red-shirts are now allies of the ruling Pheu Thai Party. Yingluck Shinawatra, Mr Thaksin's sister, led the party to a landslide victory in July 2011 and became Thailand's first woman prime minister.