Short answers, pulled from the story.
The Ukrainian Insurgent Army, or UPA, carried out the massacres, with support from parts of the local Ukrainian population. The killings targeted the Polish minority in Volhynia, Eastern Galicia, parts of Polesia, and the Lublin region from 1943 to 1945.
The massacres took place from 1943 to 1945, with the deadliest period in July and August 1943. The single bloodiest day, the 11th of July 1943, is remembered by many Poles as Bloody Sunday, when the UPA attacked about 100 Polish villages.
Up to 100,000 Polish civilians were killed, according to Poland's Institute of National Remembrance, which breaks the total down to 40,000-60,000 in Volhynia, 30,000-40,000 in Eastern Galicia, and at least 4,000 in Lesser Poland. Ukrainian civilians also died in retaliatory attacks by Poles, estimated at 2,000-3,000 in Volhynia alone.
The ethnic cleansing was a Ukrainian attempt to prevent the postwar Polish state from reclaiming sovereignty over Ukrainian-majority areas that had been part of prewar Poland. UPA commander Dmytro Klyachkivsky issued an order in June 1943 calling for the general physical liquidation of the entire Polish population.
Yes. Poland's parliament adopted a resolution in 2008 calling the UPA's crimes against Poles crimes bearing the hallmarks of genocide, and strengthened that language in 2013 to ethnic cleansing with the hallmarks of genocide. Ukraine and some non-Polish historians dispute this and describe the events as ethnic cleansing instead.
On the 11th of July 1943, UPA units surrounded and attacked about 100 Polish villages in Volhynia, burning them and killing an estimated 8,000 Polish men, women, and children, including hospital patients and nurses. The attacks that day were part of a broader wave that struck 520 Polish villages across July 1943 alone.