I will point you to this answer I wrote on the role of Gorbachev in the dissolution of the USSR. In essence, Gorbachev, after becoming General Secretary of the Communist Party in 1985, undertook a series of structural political and economic reforms that unleashed forces that were ultimately outside of his control. Thus while the formal dissolution of the USSR is taken to be the Supreme Soviet voting for its own abolition and the resignation of Gorbachev as Soviet president and the lowering of the Soviet flag over the Kremlin on December 25, 1991, this was largely the remnants of the Soviet government recognizing the already-existing state of the Union's dissolution (and it should be noted that some Soviet institutions, notably its military, actually persisted after this date).

In answer to your side-question - the Communist Party of the Soviet Union was stripped of its constitutional legal monopoly in March 1990, and was more or less dissolved and outlawed following the failed August 1991 coup. Lower level elements of the party reconsituted themselves into various political parties across the former USSR, and the largest since 1993 has been the Communist Party of the Russian Federation. The CPRF has contested elections since 1993, winning a majority of deputies to the Russian legislature (Duma) in 1995, and its leader Gennady Zyuganov almost won the 1996 presidential election against Boris Yeltsin. Despite all this, and despite being a pro-government party, it hasn't actually controlled any part of the Russian government above the regional level in the post Soviet period.