Everybody knows about NBA runs. Every team has them, multiple times per game. Much rarer in the NBA is the defensive run. That's when one team suddenly can't get a shot off, for several minutes in a row. Not many teams in history have been capable of regularly putting on defensive runs. The 2015-16 Celtics are doing it. They've done it in each of their last three games, and it seems likely they'll continue for some time to come.
Standard (offensive) runs generally happen because when Team A scores, Team B must bring the ball in from out-of-bounds.