The term comes from baseball - small ball is getting a lot of base hits that eventually become runs - like the Kansas City Royals - VERSUS long ball, which is a team hitting a lot of home runs.
Small ball - marathon, long haul poker (essential for multi-day tournaments)
Long ball - sprint, short term poker
So a long ball poker player tends to risk his entire stack a LOT more than a small ball player who risks a little to accumulate chips over the the long run. Small ball is the ideal strategy for big, long, slow tournaments, like the WSOP
Main Event. For small, short, fast tournaments then long ball comes more into play - there just isn't time for small ball.
Inherently the small ball player will need to see more flops than the long ball player. Because of this you need a couple of skills to make it work - you must be able to have the discipline to give up and fold when a hand goes south on you. This means you have to be excellent at having a read on your opponent. Basically you need to be a skilled and experienced player. You must also have fearless aggression.
Long ball = risking a lot to make a lot.
Small ball = minimizing risk, using less chips, to see more pots.
Rayer does an excellent introduction to Small Ball and Negreanu goes into some technique overviews (BELOW):