This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
"Input sensitivity" is the SPL the driver will produce given one watt
of power as measured from one meter away given some input frequency
(usually 1kHz unless otherwise noted on the speaker). Typical
sensitivities for car audio speakers are around 90dB/Wm. Some
subwoofers and piezo horns claim over 100dB/Wm. However, some
manufacturers do not use true 1W tests, especially on low impedance
subwoofers. Rather, they use a constant voltage test which produces
more impressive sensitivity ratings.
"Frequency response" in a speaker refers to the range of frequencies
which the speaker can reproduce within a certain power range, usually
"Impedance" is the impedance of the driver (see Section 1.1), typically
4 ohms, although some subwoofers are 8 ohms, some stock Delco speakers
are 10 ohms, and some stock Japanese imports are 6 ohms.
"Nominal power handling" is the continuous power handling of the
driver. This figure tells you how much power you can put into the
driver for very long periods of time without having to worry about
breaking the suspension, overheating the voice coil, or other nasty
"Peak power handling" is the maximum power handling of the driver.
This figure tells you how much power you can put into the driver for
very brief periods of time without having to worry about destroying it.