In order to make a fermentable liquid with barley, you have to malt the grain first. Malting involves partially sprouting or germinating the barley kernels, then drying them in a kiln. What this does is activate enzymes that are already in the grain that start to break down the long starch chains within the grain kernel to shorter starch chains. When the sprouted grain is kilned, this process is stopped. The resulting product is malted barley or just "malt". This is the basic malting process. The amount of kilning that a malt gets depends on the type of malt you are trying to produce and the specific barley strain that was originally malted. Pilsner malt is very lightly kilned, Munich malt is kilned a bit more. The more it is kilned, the darker the grain. The malting and kilning processes in conjunction with other processes like "stewing" and "roasting" can be tailored in many different ways to get many different types of malt. But this is the basic malting process.
Once the grain is malted, the enzymes within the grain are still present, but not active. When the malt is crushed and then introduced to water at a specific temperature (mashing), the enzymes become active again and start converting the shorter starch chains in the grain to simple sugars which are then later fermented by the yeast. There are two basic enzymes in malt, Alpha Amylase and Beta Amylase. These exist in the barley kernel when it is grown. Their job is to covert starch to simple sugars for the plant to use during sprouting and growth. Each one has it's own characteristics and breaks down the starches a little differently. In addition, each one has it's own particular behavior within a different temperature range. Depending on the type of beer you brew, the selection of a mashing temperature or multiple mashing temperatures will yeild a different sugar profile for the yeast to consume. In general, lower mashing temperatures (150 F +/-) yeild a more fermentable sugar profile, resulting in dryer beers, and higher mashing temperatures (155 F +/-) yeild a less fermentable sugar profile resulting in a maltier/sweeter beer. These are only broad generalities.
A quick Google search on "malting" or "what is malting" will yeild many sites and essays that give more info that I can give in a 2 paragraph response.
Prosit,
. But one year, when I have lots of free time on my hands I am going to try it. How cool would it be to say; "Hey, wanna' try some of this beer I grew?" 
