A speaker's power rating (in general) tells you how much A.C. power can be dissipated in the speaker's voice coil without damaging the speaker.
Honest Power Ratings:
The most honest way to rate a speaker is to give the rating as continuous RMS watts (ex: 150 watts continuous RMS power). You may hear speakers advertised as "150 watt 6 x 9s" or "100 watt 6 x 9s" and you may instantly think that the "150 watt 6 x 9s" are better and will play louder than the speakers rated at 100 watts. The first thing you should realize is that speaker ratings are OFTEN exaggerated. Then you should ask if the rating is in RMS or peak watts and are the speaker ratings for instantaneous or continuous power. Most car audio speakers (with the exception of some subwoofers) are rated in peak power or music power. Only a few speakers (generally the higher quality speakers) are rated in RMS watts. While peak power is a legitimate way to rate speakers (as long as the manufacturer tells you that the power rating is in peak watts), it can be deceptive.
Peak vs RMS:
You know that peak power is 2*RMS power. If a speaker is actually capable of handling 150 watts of peak power it would only be rated to handle 75 watts RMS. If a speaker is rated to handle 150 watts 'music power', it may mean that the speaker will take only very short bursts of power approaching 150 watts RMS. Even if there are two speakers from different manufacturers which have the same power ratings, one of the manufacturers may be more conservative in their ratings than the other manufacturer. The more conservatively rated speaker would be more likely to handle its rated power. Bottom line, beware of power ratings on speakers. Knowing that some manufacturers are less than honest in their power ratings, will likely help you to make better decisions when buying speakers.
Amplifiers and Speakers:
Many people ask the question... Can my speakers handle this amplifier or will this amplifier blow my speakers. Well, the truth is that any speaker can be driven by any amplifier. The only time that there will be a problem is when the person operating the system becomes abusive. Most people (and I do mean most) drive their amplifiers well into clipping. I know what your thinking... I never drive my amp into clipping. Well, you must be one of the very few. Generally speaking, if you have friends who are impressed by high volume, you drive your system into clipping.
Unless you listen to your system from a distance, you cannot hear clipping distortion until it reaches extreme levels. I know you are thinking that you can probably hear even .5% distortion levels in music. Well, if you were listening to a test tone in an anechoic chamber at an 80 dB SPL on your best day, MAYBE. But... due to the design of the human ear, you cannot hear minimal distortion levels at higher SPLs. At any SPL above approximately 90 dB, your ears overload and cannot accurately convert the sound pressure to the electrical impulses which are sent to your brain. Since most amplifiers are capable of producing more than one watt of power and most speakers will produce at least 88 dB of sound pressure at one watt of input at one meter, it is very difficult to hear minimal distortion at 10, 20, 50 or more watts. If you honestly want to see if you're driving your system into clipping, play some familiar music at the highest volume that you would play it (when you are showing off for your friends) and step way away from you vehicle (with the doors open of course). You may have to turn off your bass amplifiers to listen for distortion in your 'highs'. I think you will be surprised at the levels of distortion that you hear. Now, be honest if you do this little experiment.
No one can tell you if you will blow your speakers with a given amplifier. They may be able to tell you whether or not a pair of speakers will be able to handle a given amount of continuous RMS power. But... since they don't know your listening habits or your ability to hear (or even be concerned about) distortion, they (in my opinion) cannot actually tell you if a given amplifier (with you at the volume control) will blow your speakers.
For more information on continuous RMS power, see the bottom of the Measuring AC Voltage page of the site.