Durometer is the scientific word for hardness in elastomers. It’s measured from 0 to 100, with 100 being the hardest.* We consider 90-97.5a to be hard, 80-88a to be Medium hard, 70-78a to be medium soft, and 60-68a to be soft. Our softest bushings can be “squished” by hand and feel a bit like a gummy bear (not as tasty though - do not try to eat our bushings).
We have the widest range of durometers of any bushing manufacturer because different disciplines and riders want different hardness's. We started RipTide because when Jeremy was young, he wasn’t heavy enough to turn his skateboard and it wasn’t fun for him. Going down to 60a gave him and a lot of other young riders the ability to turn and control their boards.
Along that line, lighter riders typically want softer (lower durometer) bushings because they take less weight to turn, and heavier riders typically want harder (higher durometer) bushings. We recommend you look at our weight charts to give you an idea of what each durometer will feel like depending on your weight, especially when combined with different shape combos.
Roller Skate now has specific weight charts since most riders need softer durometers since their weight is divided across two feet, and Roller Skate hangers are smaller. Smaller hangers provide the rider with less leverage to turn, and softer bushings can make up for that. The reverse also applies, where larger hangers like in Electric Skateboards usually need higher duros, which is why we almost exclusively recommend KranK for E-SKate. 
APS™ Formula: Animated Polymer System
APS is our most lively feeling urethane with high rebound and lean. It will turn 10-15% more than the same durometer of other brands. It has the widest range of durometer from 97.5a (super hard) down to 60a (really soft), and is a great starting place if you're new to changing your bushings.
KranK™ Formula: Performance Under Pressure
KranK is our highest rebound formula and has a range from 96a to 84a. It will feel harder the tighter the kingpin nut is screwed on - allowing for customization of feel within the same bushing! KranK will turn less than the same durometer of other brands.