Amplifier

if you have 90x2 (90 watts by 2 channels for total of 180 watt amp) you will have 90 watts in to two channels assuming that the load on the amp is 4 ohm on each channel.. also, watch the voltage.. some will put something like:

90 x 2 4ohm @ 12v

120 x 2 4 ohm @ 14.4v

240 x 2 2 ohm @ 14.4v

One amp that is VERY misleading as to the power is the new rockfords... that new Punch 150 is crap.. 37.5 watts a channel.. the 150 watts is rated at 2 ohm stereo (4 ohm Mono)  so your new punch 150 is really like an olf punch 75... But of course, we ALL know that the best amps rockford EVER made was the OLD (pre 90's) Punch 45 and the Power 650.. now those were amps.. Protection??? Awwww... put a couple of fuses in line.. That'll work..

Short answer:  Your amp is rated to deliver 90 watts utilizing a 4ohm resistance per channel.  Left channel at 4ohms and right channel at 4ohms for a total of 180 watts. 

Long answer:  What kind of amp is it?  What do you plan to power with it?  And what type of listening do you expect to achieve?

Snoman beat me to the post.  He is dead-on with respect to how power is measured by manufacturers.  Be aware of the big picture from amp to amp.  Sufficient clean power will most likely prevent you from replacing blown speakers. 

Quality of power is not so important for producing low frequency waves (subwoofer) as it is for mid to highs.  That said.  Your Fosgate will produce way more relative power than your Kenwood.  The Kenwood is underpowered and unclean.  You'll eventually keep crankin on that volume knob to get that addictive Fosgate bass fix while your Kenwood is screamin in agony.  Speaker failure is commonly caused not by overpowering your speakers, but instead by not providing enough juice.  The oldschool Kenwood will get you by if you set your system up relative to power output and youre disciplined on the volume knob.  Kind of like having a Muscle-car with a speed-limiter.