Mashing is the process that converts the unfermentable starches in malt to fermentable sugars. It is done by adding a certain amount of hot water to the grains in a vessel referred to as a mash tun. There are enzymes naturally present in the malt that will work to convert the contents of the barley kernel to fermentable sugars. These sugars fall into three categories: monosaccharides, disaccharides, and trisaccharides.
Monosaccharides are carbohydrates comprised of just one molecule. The following are monosaccharides relevant to brewing: glucose, fructose, galactose, and mannose.
Disaccharides are carbohydrates comprised of two molecules. The brewing-relevant disaccharides are: maltose, sucrose, lactose, and melibiose.
Trisaccharides are carbohydrates comprised of three molecules. The brewing-relevant trisaccharides are: maltotriose, glucodifructose, fructosant, and raffinose.
Some of these sugars are already present in the malt at the outset. Others will have to be produced by the breakdown of starches by various enzymes in the mash. Of all of these, maltose is arguably the most important. According to Fix, it constitutes between "46 to 50% by weight of the sugars in a grain wort."
The production of maltose in a given wort is largely determined by the enzymatic action of α-amylase and β-amylase. They both produce maltose, though α-amylase also produces other sugars.
Imagine a large group of inflated balloons tied closely together with string. Now imagine that you get to throw darts at that group of balloons. But these are very special darts. In fact, there are two types of darts. The first group of darts act in such a way that each can only pop one balloon at a time. After these darts break two or three balloons however, they become incapable of breaking more. The second group of darts is capable of releasing many balloons at one time (that is, making smaller clusters of balloons) as well as breaking just one balloon.
If you used just darts from the first group, you would break many balloons, but leave behind the core of the clusters. If you used just the second group, you would get many smaller clusters, but not really break all that many balloons. Obviously to break the most balloons, you would have to use both types of darts.
In the above metaphor, the group of balloons is a large carbohydrate (often referred to as an Amylopectin), β-amylase is the first group of darts and α-amylase the second. Because both of the enzymes have different optimal temperature ranges, a compromise between temperatures must often be reached in order to mash successfully. Specifically, β-amylase works best between 131-150F and α-amylase works best between 154-162F. Palmer goes on to note that, "The temperature most often quoted for mashing is about 153°F."
The take away message is this: If you want to make a drier, thinner, more fully attenuated beer, mash a bit lower than 153F. This type of mashing favors the β-amylase enzyme. If you want to make a somewhat sweeter, fuller-bodied, less-fully attenuated beer, mash a bit higher than 153F. Hopefully now it is easier to see why this is the case.
 Noonan, Gregory J. New Brewing Lager Beer (Brewers Publications, 1996)
 Fix, George Principles of Brewing Science (Brewers Publications, 1999)
 Palmer, John How to Brew 1st Edition (http://www.howtobrew.com/section3/chapter14-5.html) Accessed on 6-26-07