Long balls are those shots which would only bounce once on the table if left. Whereas short balls would bounce twice.
If a ball lands short, it's very hard to attack so shot options are limited. Whereas long balls give you lots of space for more powerful shots to be played (drives, loops, chops).
All attacking shots will land long. Most backspin shots will land short. However, lower quality chops and chop serves will land long.
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To know for sure if a ball will land short, look out for the dashed line on the table. If you don't see this, you know it's a long ball.