Ocean waves breaking on the beach commonly arrive in sets.  A
wavetrain with say 5-10 more or less evenly spaced waves will be
followed by a quiet spell, then another set.  Why?
Dispersion may seem to offer an explanation (it has been offered to me
before in response to this question): longer waves travel faster, so
waves get sorted by wavelength as they traverse the ocean from their
generation point, and the long swell from a storm may reach shore
hours, or even days, before the shorter, slower waves.  However, this
does not, to my mind, constitute an explanation of the phenomenon, at
least not a complete one.  Unless the wavelengths stimulated by a
given storm are somehow discretized, it seems that waves from the
storm should arrive continuously, with gradually increasing
wavelength.

The phenomenon you describe is characteristic of so-called "beat"
frequencies, which occur when you add two waves of slightly different
frequencies. You get an amplitude modulation at the difference
frequency. Piano tuners use this phenomenon to help them tune strings
(they listen for the beat frequency, which sounds like a pulsing of
the sound). The same phenomenon also occurs in ocean waves (and
therefore beach waves). If slightly different frequency waves are
simultaneously present (very common) then beat frequencies will be
generated and waves will appear to come in "sets."

You can see this illustrated and discussed further at 
"Wave Motion"
http://www.media.uwe.ac.uk/masoud/projects/water/wave.htm

You might also be interested in the discussion of ocean waves and the
data on ocean waves at the National Data Buoy center:
“What causes ocean surface waves?”
http://www.ndbc.noaa.gov/educate/pacwave.shtml

If I've misunderstood the phenomena you are seeking to explain, feel
free to request a clarification.

Google Search:

ocean wave frequency
://www.google.com/search?q=ocean+wave+frequency

beach wave frequency
://www.google.com/search?q=beach+wave+frequency

beach wave beat frequency
://www.google.com/search?q=beach+wave+beat+frequency

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Request for Answer Clarification by racecar-ga on 09 Dec 2002 15:31 PST

Just a comment: found someone else's answer to the same question.

http://content.surfline.com/sw/content/community/whoknows/12_02_sets.jsp