The Communist Party of the Soviet Union was the name used by the Russian Social Democratic Labour Party following the Russian Revolution. The party had split into two factions, the Bolsheviks and the Mensheviks, primarily over the issue of party membership. Bolsheviks favored a closed membership of strictly full time professional revolutionaries; whereas the Mensheviks favored open party membership. The Bolsheviks seized power in October of 1917 (by the Julian calendar still in use in Russia at that time). Soon after, they banned the Mensheviks (and all other opposition political organizations) and changed their own name to the "Communist Party".
A name change had been first mooted in Lenin's April Theses, which articulated Lenin's sense that the term Social-Democracy had become devalued, a notion he articulated in his pamphlet Socialism and War (1915) where he talked of the pro-war Social-Democrats as Social-Chauvinists.
In February of 1990, the CPSU called for the end of its constitutional guarantee of power. In March, the Congress of Peoples Deputies repealed Article Six of the Soviet Constitution, which had guaranteed monopoly political power for the party.
Following the August coup in 1991, the CPSU was banned by Boris Yeltsin on all Russian soil. The KGB was disbanded as were other CPSU-related agencies and organisations. Yeltsin's action was later declared unconstitutional but by this time the USSR had ceased to exist.