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The NBA is split into two conferences, the Eastern Conference and the Western Conference. Both conferences were formed in 1946, and even though teams expanded, merged, and moved, these two conferrences remain the two primary divisions in the NBA.
There are 15 teams in each of the two NBA conferences. The Eastern Conference has teams such as the Celtics, the Nets, and the Knicks, while the Western Conference includes the likes of the Lakers, Warriors, Jazz, and others.
The top eight teams from each conference move on to the NBA playoffs based on their regular season record. Also, the top seed in each conference earns a first-round bye.
Each NBA Conference is split into 3 divisions, for a total of 6 divisions in the NBA. The Eastern Conference includes the Atlantic Division, the Central Division, and the Southeast Division. The Western Conference includes the Southwest Division, the Northwest Division, and the Pacific Division. Both divisions consist of 5 teams, for a total of 30 teams in the NBA.
The NBA last changed its division structure in 2004 with the expansion to include the Charlotte Bobcats (now Hornets). The 6 division structure was implemented to create better balance and competition, and to allow for more efficient scheduling and travel. Prior to 2004, the NBA had two divisions in each conference,the Atlantic, Central, Midwest and Pacific divisions.