The traditional man-to-man defense is by far the most common defense in the game of basketball today.
This is especially true as players progress to a higher level of competition. In fact, due to the great outside shooting of professional players, every NBA team almost exclusively plays man-to-man defense.
The man-to-man defense involves all 5 defensive players on the court being allocated one opposition player who they’re accountable for defending whenever they’re on defense.
Of course, there will be times when players switch opponents or help each other on defense, but for the most part, each player is responsible for one player from the opposition.