The political structures of the USSR and the CPSU could differ in their role and influence from time to time, partly, or rather largely, because Soviet political practice has always operated with relatively little formal rules or descriptions such as in the Western legal tradition. In fact, the legal tradition in that respect is rather weak in Russian history. The workings of the various organs of the political system were further complicated because of the practice of holding several positions simultaneously, thus for example being simultaneously a government minister, CC member, or Party Secretary and Politburo member.

Political power and authority often did not derive from the position or function itself, but was rather of a personal nature, and the person occupying a certain position determined how influential it was. So while KGB chief Ivan Serov was merely a junior figure within the top political hierarchy of the Soviet system, the later KGB Yuri Andropov was hugely influential through his good relationship with Leonid Brezhnev, and it was eventually affirmed by his entrance to the Politburo and the CC Secretariat after Suslov's death. Meanwhile, Nikolai Podgorny was head of the Presidium of the Supreme Soviet, the closest Soviet equivalent to being head of state, but Podgorny's position was largely symbolic, spending his days with diplomatic trips abroad and receiving foreign dignitaries.

Especially in the period 1917-1953, covering the Civil War, the factional struggles of the 1920's, and the Stalin era, the political structures of the CPSU and USSR were unstable and constantly changing and mutating. In practice the Central Committee was the main representative body of all local and regional branches of the Communist Party. These came together in a Congress once in a while, 3 to 5 years, sending thousands of delegates and taking several days. At the Congress, a more permanent body, the Central Committee, was elected and responsible for the governing of the CPSU in between congresses. This started out as a small body of some 20 deputies, but later the CC's membership varied around some 200 to 300 members. Stalin had the majority purged in the 1930's, and afterwards it never regained the authority it once held over the direction and leadership of the country. It degenerated into a large audience of Soviet officials serving as a transmission belt between the leadership and the provinces, and for approving and applauding the policies of the Soviet leadership, including its nominations for leadership positions such as the Politburo and Secretariat.

The CC seldomly convened (called a Plenum) after the Terror, usually a few times a year, listening to speeches and applauding for it. The CC was therefore more form than substance, and it wasn't a fulltime job to be CC member. It became something of an honorary title, Soviet aristocracy, combined with other influential positions in regional politics or central governance, industry, military, security, foreign policy, culture or science and academia. The networking and lobbying power that came with holding CC membership is what made these people important. In 1957 the CC played a crucial role when Nikita Khrushchev faced a revolt within the Politburo that was seeking to depose him, the so-called Anti-Party Group. Since only the CC could formally appoint the General Secretary, Khrushchev convened an emergency session of the CC which re-affirmed its support to him and expelled the members of the Anti-Party group from the Politburo. This same trick was used by his opponents however in 1964 to dismiss Khrushchev. One crucial task of the CC was to oversee the Party Control Committee, the disciplinary organ, and it could thereby initiate disciplinary investigations against other party members and expel them. For instance, during the power struggle between reformers and hardliners in the early 1980's, it appointed the hardliners Zimyanin (Propaganda Tsar) and Viktor Grishin (Moscow Party Secretary) to investigate the reform-minded CC member Inozemtsev who headed the reform-minded foreign policy think tank IMEMO. Leonid Brezhnev eventually intervened to halt the witch hunt against this personal friend.

The Politburo

The Politburo actually emerged because the CC had grown too large and impractical to handle executive affairs of the Party and State with its hundreds of deputies. Approved by the CC, its membership varied around a dozen permanent members and a dozen candidate members, often no fewer than 10, not more than 20. Although convening much more regularly, under Stalin this became increasingly rare as Stalin preferred to meet individually with each member. Its function was to review and vote on party policies and proposals and issueing decisions on policy, budgets and resource allocations. After Stalin and Khrushchev, the principle of collective leadership returned and the Politburo's sessions became more regular and standardized, becoming a sort of ''council of ministers''. The directives of the Politburo were carried out by the Secretariat, headed by the General Secretary - both appointed by the Central Committee.

The General Secretary presided over a small bureaucracy called the Apparat, divided into some 20 policy departments, overseeing some 50 Soviet government ministries. The CC Apparat developed policy drafts together with the ministries, and oversaw the implementation of Politburo decisions and personnel appointments. Roughly it consisted of some 1000 officials. To assist the General Secretary in directing the Apparat, there were usually 2 or 3 secondary Secretaries who in turn oversaw a handful of other secretaries responsible for several CC Departments. The Secretariat thus consisted of a dozen people in total. It was under Stalin that this body, then mainly responsible for personnel and Party organization, became the center of executive power and a ''state within a state''. All information of the secret police for instance flowed to the ''Special Department'' (Osobii Otdel) of the CC Apparatus, headed between 1928 and 1952 by Stalin's loyal cup-bearer Alexander Poskrebyshev.

So to conclude, the Central Committee started out as a small leading body of the Party, but grew into a large symbolic organ serving mainly to confirm the actual ruling bodies of the Party, the Politburo and the Secretariat. The Politburo's power and procedures moved back and forth depending on the leader, especially under Stalin, but it eventually settled as a sort of decision-making body similar to a council of ministers or a state council.

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I can recommend two works in particular if you want to know more about Soviet politics:

Richard Sakwa, Soviet Politics in Perspective (Routledge, 1998).

Jerry F. Hough and Merle Fainsood, How the Soviet Union is Governed (Harvard University Press, 1979).