Pimple table tennis rubbers are rubbers with pimples on the outside of the surface. These rubbers grip the ball differently, and more suited to manipulating your opponent's spin, rather than spinning the ball. Inverted rubbers (smooth on the outside) are generally better for generating spin. Pimple rubbers can be sorted into 3 different categories:
Long pimples are the most defensive style pimples and are most commonly used for chopping away from the table against loops, allowing one to control the loop and return very heavy backspin. These rubbers are also commonly used for a blocking and pushing game, where the 'spin reversal' of the rubber is used draw mistakes from your opponent, to setup the ball for a powerful finishing stroke with the inverted side.
Medium pimples have properties that are somewhere in between those of long and short, offering some of the more disruptive and spin reversal properties of long pimples, and also some of the more offensive properties of short pimples. Useful for players that play both styles, but most commonly used for a more offensive style close-in like that used for short pimple, where they offer more disruption while sacrificing some speed potential.
Short pimples are the most offensive style rubbers, as they are relatively insensitive to incoming spin, and when used for hitting almost all the power of your stroke is used for speed, so they can be even more powerful than inverted. When hitting or blocking with short pimple, they produce a different combination of speed and spin compared to inverted, so unless your opponnets adjusts to this, it can draw weak balls or mistakes. Short pimple are almost always used close-in for a fast game, and the ball hit at the top of the bounce which offer the lowest risk and offers your opponent less time to react.
