Tides are caused by the moon's gravitational pull, and the fact that it orbits the Earth
 roughly once each day.

Here is what Microsoft Encarta has to say on the subject:
 "The moon, being much nearer to the earth than the sun, is the principal cause of tides.
 When the moon is directly over a given point on the surface of the earth, it exerts
 a powerful pull on the water, which therefore rises above its normal level.
 Water covering the part of the earth farthest from the moon is also subject to this pull,
 so that another distinct dome of water is formed on the farther side of the earth
 providing the basis for a second wave. The lunar wave crest directly beneath the moon is
 called direct tide, and the crest on the side of the earth diametrically opposite is
 called opposite tide. At both crests, the condition known as high water prevails, while
 along the circumference of the earth perpendicular to the direct-opposite tidal axis,
 phases of low water occur."
"Tide," Microsoft(R) Encarta(R) 96 Encyclopedia. (c) 1993-1995 Microsoft Corporation.
 All rights reserved. (c) Funk & Wagnalls Corporation. All rights reserved.

However, on the subject of why there are two tides per day in most places on the Earth, I
 would take issue with Encarta:-
Since the moon orbits the Earth regularly, and approximately daily, this ties the number of
tides per day to be an integer (whole number), due to resonance theory. The reason there are
*two* tides per day, is not (IMHO) because of any "opposite tide" pull (which surely would 
be _weaker_ than at the sides of the Earth anyway), but simply because the closest integer
 to the number of distinct bands of alternating water (ocean) and land (masses) is two.
That is to say, going round the Earth once involves (approx) two bands of water (Pacific
 and Atlantic Oceans) alternating with (approx) two bands of land (Eurasia/Africa and The Americas).
Thus two tides per day is the most stable outcome.