When buying wheels that are side set, it means that the core of the wheel is set to the very edge of the wheel. This puts pressure onto the inside of the wheel, and this makes sure that you won't cone your wheels too fast.
Center Set Cores
Center set cores are good if you're doing a lot of sliding because the pressure of the board is directed evenly onto into the center of the wheel. This gives you lots of grip, but if your wheels start to cone you can flip them the other way and they will wear down more evenly.
Off Set Cores
In an off set wheel, the core is set off to the side a bit. These wheels give you the best of the other two cores. (side set and center set) These wheels will make sure that they don't cone and that they have plenty of grip.
3 Core Comparison
silverfishlongboarding.com
1) On the left (the flashbacks) are side set. 2) In the middle (the grippins) are center set. 3) On the right (the gumballs) are off set.