I assume you were looking for definitions of straight rate or progressive rate fork springs.
From Sonic Spring's web site:
"Straight-Rate Springs vs. Progressive Rate Springs
Over the last 20 years, one of the biggest changes in the aftermarket motorcycle suspension world has been the virtually complete switch from progressive rate springs to straight rate ones. I think itís safe to say that thereís not a single reputable suspension tuner who advocates progressive springs. The question is why? Whatís the problem with a progressive rate? Why are straight rate springs better?
One of the problems is that bikes, street bikes anyway, just donít have enough travel to take advantage of progressive rates. The soft initial portion gets blown right through, leaving a limited amount of travel for the stiffer portion to deal with. This results in less compliance, less traction and a harsher ride. Another issue is damping; Damping rates, particularly rebound damping, need to be matched to the spring rate. With a progressive (i.e. variable) rate, thatís impossible. Damping is always a compromise and a progressive rate just makes the balancing act that much more difficult. Adding to the problem is that modern forks actually have 2 spring mediums, the steel coil and the air trapped inside. The air is intrinsically a highly progressive spring. Adding a progressively wound steel spring to the mix is just making a bad situation worse."
From Race Tech's web site:
"Q: I have heard of progressive springs and this concept makes sense to me. Why does Race Tech recommend Straight Rate Springs for forks?
A: When setting up the spring forces in a bike you want a setup that is progressive enough yet not too progressive. A setup that is not progressive enough will have a tendency to feel too harsh yet when a big hit is encountered bottoming occurs. A setup that is too progressive will either drop through the travel feeling mushy and then feel as though it hits a wall or can feel good until it hits that wall. On front forks there are two forces we consider to be "spring" forces. First is the coil (mechanical) spring and second is the force due to air pressure and oil level. Even if you run zero air pressure the oil level causes the pressure to increase as the forks are compressed. By its very nature this increase in pressure is very progressive. We have found that in combination with a straight wound spring we have a good level of progressiveness. If we want more progressiveness we simply raise the oil level.
Anolther subtle benefit of straight wound springs is that they are easy to understand. In order to make sense of progressively wound springs you really have to map out the force as you compress the spring. For example a spring marked 20/40 lb/in (excuse me for the SAE units but this will work with metric units as well) may start out at 20 lbs/in in the first inch but where does the 40 refer to? It might be referring to the rate in the 4th inch or it could be referring to the rate in the 6th inch. This would cause a huge difference in ride."
From Direct Parts web site:
"Progressive rate fork springs have many advantages over straight rate springs. A straight rate spring only offers linear resistance to compression and is a compromise, too stiff for small bumps and too soft for large ones. A Progressive rate spring is wound tighter at the top so larger bumps cause the spring to resist the greater load."
Race Tech also has a spring rate calculator:
Spring rate calculator