This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
Wiring speakers in series involves connecting at least two speakers so
that the first speaker's positive lead is connected to the amplifier's
positive terminal, and the negative lead is connected to the positive
lead of the second speaker. If there is a third speaker, its positive
lead will be connected to the second speaker's negative lead ... and so
on. The last speaker in the chain will have its negative lead
connected to the amplifier's negative terminal.
Speakers that are wired in parallel are all connected to the positive
and negative terminals of the amplifier. So, when two speakers are
wired in parallel, you'll connect each speaker's positive lead to the
amplifier's positive terminal, and you'll connect each speaker's
negative lead to the amplifier's negative terminal.
Be careful when wiring multiple speakers in parallel or series so that
you do not exceed your amplifier's rating. To calculate the effective
impedance of a number of speakers, use the following formulas:
Z(t) = Z(1) + Z(2) + Z(3) + ... + Z(n)
That is, add up all of the impedances for each speaker to
get the total impedance. For example, with 3 4-ohm speaker
in series, the total impedance is 4 + 4 + 4 = 12 ohms.
1/Z(t) = 1/Z(1) + 1/Z(2) + 1/Z(3) + ... + 1/Z(n)
That is, add up the inverse of the impedance of each
speaker and invert the sum to get the total impedance. For
example, with 3 4-ohm speakers in parallel, the total
impedance is 1 / ( 1/4 + 1/4 + 1/4) = 1 / (3/4) = 1.33 ohms.