Short answers, pulled from the story.
Josip Broz Tito led the Yugoslav Partisans throughout World War II. He organized the movement on the initiative of the Communist Party of Yugoslavia following the Axis invasion of Yugoslavia in April 1941, and commanded the movement through its growth from a guerrilla force into an army of over 800,000 by 1945.
By late 1944 the Partisans numbered around 650,000, organized in four field armies and 52 divisions. By April 1945 the force exceeded 800,000 soldiers grouped in 63 divisions. They began the war as a small, poorly armed guerrilla force with around 81,000 fighters recorded in late 1941.
According to pension records from 1977, the ethnic composition over the entire war was 53 percent Serb, 18.6 percent Croat, 9.2 percent Slovene, 5.5 percent Montenegrin, 3.5 percent Bosnian Muslim, and 2.7 percent Macedonian, with the remainder including Albanians, Hungarians, Italians, and others. More than 40,000 Italian fighters served in formations including the Division Garibaldi. The Serb majority in the early war years shifted substantially after Italy's capitulation in September 1943.
Allied support shifted in late 1943 after British intelligence missions confirmed that Chetnik forces were collaborating with the Axis. Brigadier General Fitzroy Maclean was parachuted to Tito's headquarters in September 1943 as a permanent liaison, and the Tehran Conference that year granted the Partisans official Allied recognition. Churchill stated in November 1943 that the 222,000 followers of Tito were holding as many Germans in Yugoslavia as combined Anglo-American forces were holding in Italy south of Rome.
The Battle of Poljana, fought on the 14th and the 15th of May 1945 near Prevalje in Carinthia, is considered the last battle of World War II in Europe. Yugoslav Partisans fought retreating Wehrmacht and collaborationist forces there. Earlier in the same final offensive, the Partisans reached Trieste two days before the Western Allies.
From the 7th of July 1941 to the 16th of May 1945, 245,549 Partisans were killed in action, 399,880 were wounded, 31,200 died from wounds, and 28,925 were listed as missing. Ivo Goldstein records that 82,000 Serbs and 42,000 Croats were killed on NDH territory as Partisan combatants.