The start of the NBA season technically is in late September. The games played from then until the middle or late end of October are preseason games.
These are useful for coaches and staff to evaluate how their players work together, especially because most teams undergo changes during the off season.
The official, actual start of the regular NBA season is mid to late October. This year, 2017, the first regular season game was on Tuesday, October 17.
There are a total of 26 weeks before the post season starts, usually sometime in early April. The postseason series are best of 7 (the team that wins needs to win 4 out of 7 games).
The series pits the winners of each of the Western and Eastern Conferences together, and happens sometime in June.