I had the same question when I started. Here's a short note about it...lots of other stuff written and stickied throughout the site.
Different manufacturers note their springs differently.
Some springs are noted in Meters Per Second (M/s)...typically with a "M" in front of a number. I.e. M90/M100/M120
Some springs are noted in Percentage %....i.e. 100%, 150%, 200%
Some springs are noted otherwise...i.e. Modify notes their springs with "S", S90+, S100+....and in Modify's case it's the same as M/s
The baseline of spring power is Tokyo Marui's stock power level which was approx 290fps (feet per second measured with 0.20g BBs). That is approximately a M90 spring.
Approx 1M/s = 3.3fps...so M90= 90M/s = 90 * 3.3 = 297fps
M100 =~ 330fps, etc....
TM's stock M90 spring is considered 100%...so a 150% spring is 90M/s * 150% = 135M/s = 135 * 3.3 = 445.5fps. (give or take 10fps)
The fuzzy part about it all...is that those numbers are HUGELY impacted by other parts that you may have in your mechbox/rifle...and the condition/fit of those parts. A bearing spring guide might add 5fps...might add 10fps with a different spring. A tightbore might add 5fps in one barrel length and add 10fps in a different tightbore barrel length. If you piston head seals badly or your nozzle leaks you might not get the full potential out of a given power spring to begin with...etc.etc.etc. Welcome to fiddling with AEGs.
Compounding all that...the way/consistency some manufacturers do their springs may mean that while they label one spring M100 and another M120....their M100 might not be the same as another manufacturers M100....and their M120 might not be "20M/s faster" than their own M100! Stick to trusted brands of springs to minimize those headaches.
A great read is Skruface's What to upgrade or not (stickied)...Greylock's stickied thread as well. Illusion's spring chart has been stickied here and there all over the Internet.
A general rule of thumb with a decently working rifle is...stick to a M90/M100 if you want to stay under indoor limits. A M110/M120 will put you very close and maybe over outdoor limits.
*edit* or what Stalker said
(but look at the top of the Upgrades section for Skruface/Greylock/Illusion's stickied threads)