They Tamil ethnic group wanted self rule in the island. They started a civil war in 1983.
The civil war lasted for just over 25 years. There has been conflict in the region for many hundreds of years. The Tamil stepped up to try and take the government when there numbers were highest and still rising.
BBC: Sri Lanka+Tamil Tigers: Evolution of the Ethnic War 4x6
What took Place?
There was open violence in Sri Lanka.
How many people affected?
Between 80 and 100 thousand people were killed in the 25 years of civil war. The whole country was affected even if they were not killed themselves. Over 250 thousand Tamil were captured in the post war and some of those rebels were killed and most were sent to prison.
The UK was involved in the conflict. The violence damaged the economy and harmed tourism in one of South Asia's potentially most prosperous societies. It was manifested when the British got involved and helped to end it.