Organisation of the Communist Party of the Soviet Union
The organization of the Communist Party of the Soviet Union was based on the principles of democratic centralism.
The governing body of the Communist Party of the Soviet Union (CPSU) was the Party Congress which initially met annually but whose meetings became less frequent, particularly under Joseph Stalin. Party Congresses would elect a Central Committee which, in turn, would elect a Politburo. Under Stalin the most powerful position in the party became the General Secretary who was elected by the Politburo. In 1952 the title of General Secretary became First Secretary and the Politburo became the Presidium before reverting to their former names under Leonid Brezhnev in 1966.
In theory, supreme power in the party was invested in the Party Congress. However, in practice the power structure became reversed and, particularly after the death of Lenin, supreme power became the domain of the General Secretary.
Higher levels 
In the late Soviet Union the CPSU incorporated the communist parties of the 14 constituent republics with the exception of Russian SFSR. The RSFSR did not have its own party organization, and the communist parties of its oblasts, autonomous republics and some other major administrative units subordinated directly to the CPSU Central Committee.
Lower levels 
At lower levels, the organizational hierarchy was managed by Party Committees, or partkoms (партком). A partkom was headed by the elected "partkom bureau secretary" ("partkom secretary", секретарь парткома). At enterprises, institutions, kolkhozes, etc., they were called as such, i.e., "partkoms". At higher levels the Committees were abbreviated accordingly: obkoms (обком) at oblast (zone) levels (known earlier as gubkoms (губком) for guberniyas), raikoms (райком) at raion (district) levels (known earlier as ukoms (уком) for uyezds), gorkom (горком) at city levels, etc.
The same terminology ("raikom", etc.) was used in the organizational structure of Komsomol.
The bottom level of the Party was the primary party organization (первичная партийная организация) or party cell (партийная ячейка). It was created within any organizational entity of any kind where there were at least three communists. The management of a cell was called party bureau/partbureau (партийное бюро, партбюро). A partbureau was headed by the elected bureau secretary (секретарь партбюро).
At smaller party cells, secretaries were regular employees of the corresponding plant/hospital/school/etc. Sufficiently large party organizations were usually headed by an exempt secretary, who drew his salary from the Party money.
Main offices 
General Secretary of the Communist Party of the Soviet Union
Became synonymous with leader of the party under Joseph Stalin.
Secretariat of the CPSU Central Committee
Leading body within the Central Committee. Headed by the General Secretary or First Secretary.
Central Committee of the Communist Party
The governing body of the Party between each Congress. Conducted the day-to-day business of the Party and the government.
The oversight body of the Party in between Party Congresses. Usually gathered once a year.
Congress of the CPSU
The gathering of Party delegates every five years. It was the oversight body of the entire Party, in theory.
See also