Short answers, pulled from the story.
More than 3,500 people were killed during the Troubles. Of those deaths, 52% were civilians, 32% were members of the British security forces, and 16% were members of paramilitary groups. Republican paramilitaries were responsible for 60% of total deaths, loyalist paramilitaries for 30%, and security forces for 10%.
The Troubles grew from a political and nationalist struggle over the status of Northern Ireland , whether it should remain part of the United Kingdom or join a united Ireland. The immediate trigger was a civil rights campaign by the Northern Ireland Civil Rights Association against discrimination in housing, employment, and voting rights. That campaign was met with violence from loyalists and suppression by the RUC, escalating into three decades of armed conflict.
Bloody Sunday occurred on the 30th of January 1972 in Derry, when soldiers of the 1st Battalion, Parachute Regiment shot dead thirteen unarmed men at a civil rights rally. A fourteenth man died of his injuries months later, and fifteen other civilians were wounded. It was the largest number of civilians killed in a single shooting incident during the Troubles, and it significantly increased Catholic support for the Provisional IRA.
The Troubles are generally considered to have ended with the Good Friday Agreement of 1998. The Agreement restored self-government to Northern Ireland on a power-sharing basis and included commitments to civil rights, police reform, paramilitary disarmament, and early release of paramilitary prisoners. Both IRA and loyalist paramilitary ceasefires preceded the Agreement, beginning in 1994.
Bobby Sands was a Provisional IRA prisoner in the Maze prison who led the 1981 hunger strike demanding restoration of political status for republican prisoners. While on hunger strike, he was elected to Parliament on an Anti-H-Block ticket. He died of starvation, becoming the first of ten republican prisoners to die in the strike. Over 100,000 people attended his funeral mass in West Belfast.
The Omagh bombing of August 1998 killed 29 civilians, the most killed by a single bomb during the Troubles. It was carried out by the Real IRA, a splinter group that rejected the Good Friday Agreement. The bombing widely discredited dissident republican groups in the eyes of many who had previously supported the Provisional IRA's campaign.