The traditional system of categorizing playing styles into attackers, blockers, and defenders is based mainly on the speed the ball is hit with, and to a lesser extent the spin put on the ball.
Attackers are those players who are hitting the ball hard and fast, and who use a range of spin from heavy topspin to very little topspin. Attackers will use backspin pushes to set up their attacks. Attackers can play from close to the table (the flat hitters) to up to 6 or 7 feet behind the table (the topspinners or loopers).
Blockers typically use little spin or speed themselves, and rely on using the opponent’s pace and spin against them. Their strokes are short and compact, and they play close to the table where their style is most effective. They will occasionally attack, or use pushes to make it difficult for their opponent to attack well.
Defenders are the players who use backspin to return the ball most of the time. They are content to wait for their opponent to make a mistake when attacking, and will only attack occasionally themselves. The distance they play from the table can range from close (the pushers) to 10 feet or more behind the table (the choppers).
This system of categorizing styles is still common today, so when you hear others talking about a particular player being an attacker, blocker, or defender, you can be sure that they are referring to the traditional styles.
Another method of describing player styles has also become more popular in recent years. This method uses the level of aggression of a player to help explain his overall playing style.