Short answers, pulled from the story.
The Molotov-Ribbentrop Pact was signed in Moscow on the 24th of August 1939, though backdated to the 23rd of August 1939. It was signed by Soviet Foreign Minister Vyacheslav Molotov and German Foreign Minister Joachim von Ribbentrop.
The secret protocol divided Poland, Romania, Lithuania, Latvia, Estonia, and Finland into German and Soviet spheres of influence. Finland, Estonia, and Latvia were assigned to the Soviet sphere; Poland was to be partitioned along the Pisa, Narew, Vistula, and San rivers; Lithuania was initially assigned to Germany but transferred to the Soviet sphere by a second protocol in September 1939.
The Soviet Union turned to Germany after tripartite negotiations with Britain and France collapsed. Britain refused to guarantee Soviet demands regarding the Baltic states, Poland would not allow Red Army troops on its territory, and British negotiator Admiral Drax had no written authority to commit to anything. Germany offered concrete territorial concessions in a secret protocol that the Western powers would not match.
Germany unilaterally terminated the pact at 03:15 on the 22nd of June 1941 by launching Operation Barbarossa, the invasion of the Soviet Union. Within six months the Soviet military had suffered 4.3 million casualties and three million more were captured.
The Soviet Union officially acknowledged the secret protocol in December 1989, when a commission headed by Alexander Yakovlev at the behest of Mikhail Gorbachev confirmed its existence to the Congress of People's Deputies. The Soviet copy of the original document was declassified in 1992 and published in early 1993.
Joachim von Ribbentrop was convicted of war crimes at the Nuremberg trials and executed in 1946. Vyacheslav Molotov died in 1986, having categorically denied the existence of the secret protocol until the end of his life.