Short answers, pulled from the story.
Ukrainian president Viktor Yanukovych refused to sign an association agreement with the European Union on the 21st of November 2013, which triggered a protest movement that grew into the largest democratic mass movement in Europe since 1989. This decision led to the Revolution of Dignity and removed Yanukovych from power following a majority vote in the Verkhovna Rada.
Russia actively supported separatism using high-level actors like Kremlin advisers Vladislav Surkov and Sergey Glazyev who organized pro-Russian protests where attendees included Russian citizens crossing the border. Ukraine's police and border guards denied entry to more than 8,200 Russians between 4 and the 25th of March 2014 while Vladimir Putin later confirmed these troops were Russian soldiers.
A referendum regarding joining the Russian Federation resulted in an official turnout of 83 percent and a 96 percent affirmative vote after the Crimean Parliament declared independence from Ukraine on the 17th of March. Russia and Crimea signed a treaty of accession on the 18th of March 2014 which was ratified on the 21st of March by a 30 gun salute under an executive order of the Russian President.
On the 12th of April a group of masked militants led by former officer Igor Girkin captured the executive committee building, police department, and SBU office in Sloviansk. Ukrainian Internal Affairs Minister Arsen Avakov labeled these gunmen terrorists and swore to use special forces to retake the buildings.
On the 2nd of May anti-Maidan protesters attacked pro-government demonstrators with batons and helmets before retreating to and occupying the Trade Unions House where militants shot at and tossed rocks at protesters below. The building caught fire resulting in 43 deaths total while teams from the Organization for Security and Co-operation in Europe assisted internally displaced persons who resettled in western Ukraine.