Short answers, pulled from the story.
The invasion of Yugoslavia began on the 6th of April 1941, when German-led Axis forces attacked the Kingdom of Yugoslavia. It ended after eleven days of fighting when Yugoslavia signed an unconditional surrender on the 17th of April, effective at noon the following day.
Germany invaded after a Yugoslav coup d'état on the 27th of March 1941 overthrew Prince Paul's government, which had just joined the Axis. Adolf Hitler took the coup as a personal insult and ordered Yugoslavia be treated as a hostile state through Führer Directive No. 25.
Germany led the invasion of Yugoslavia with support from Italy, Hungary, Romania and Bulgaria. Germany fielded 19 divisions and over 750 aircraft, Italy committed 22 divisions across its 2nd and 9th Armies, and Hungary's 3rd Army joined the ground offensive on the 11th of April.
Germany took between 254,000 and 345,000 Yugoslav prisoners during the invasion, while Italy captured another 30,000. German losses over twelve days of combat totaled 558 men, including 151 killed.
Colonel Draža Mihailović organized resistance in the mountains of Serbia and Eastern Bosnia after Yugoslavia's surrender on the 18th of April 1941. King Peter II's government-in-exile made him a General and Minister for War, and his guerrilla force grew to as many as 100,000 soldiers, though he was widely known as the leader of the Chetniks.
Yugoslavia was partitioned among the Axis powers, with Germany controlling most of Serbia while Hungary, Italy and Bulgaria annexed other regions. Croatia became the Independent State of Croatia, proclaimed by Ustaše leader Ante Pavelić, though Germany and Italy jointly controlled it in practice.