Wine bottles can be stored either horizontally or tilted at an angle (between 0 and 45 degrees). However, for various reasons, the most preferred orientation for storage is at an angle.
Storing them horizontally or at an angle allows the wine to be in contact with the cork. This keeps the cork damp; prevents the cork from drying up.
However, an advantage of storing wine bottles at a certain angle is to take into consideration the expansion and contraction of the headspace (the ullage, or the gas) of the wine when the wine bottle is exposed to changing temperatures. An increase in temperature will cause an expansion of the gas. If stored horizontally, the expansion might push wine into cork and out. During compression or contraction, the reverse happens. The result is that oxygen from the atmosphere might be pulled in (sucked into) the bottle. Increased oxygen levels would mean a more rapid oxidation and maturation process which is not preferred. Meanwhile, if stored at an angle, this cannot happen -- no oxygen cannot get into the bottle.
A temperature fluctation of 10 degrees celcius is enough to cause this "breating" of the bottle, which makes it necessary to store the wine properly should you expect such fluctuation in temperature.