With the prominence of HBO's Boardwalk Empire, it seemed like an interesting exercise to determine how people drank during Prohibition, because it's clear that the flow of alcohol was dampened rather than cut off.
Prohibition Agents ranged from inept to corrupt, as agents often used their training to flip to the dark side (much like ex-military working for Mexican drug gangs).
Onto the sources of booze during the "Dry Age," circa 1920-1933.
Circa because certain states took longer than others to adopt the amendment as state law.
In order from largest sources to the smallest:

1. Imported Liquor - Canadian Club whiskey was initially manufactured in the United States and called "Club Whiskey.
   " At some point its founder moved factories across the Detroit River to Windsor, Ontario.
   Eventually the name changed as well.
   During United States prohibition, Canadian Club massively ramped up production and with Detroit's proximity to Chicago, it was Capone's main business.
   Why buy whiskey in Atlantic City if it was being made less than 300 miles away? The other main import was rum from the Caribbean.
   So-called rum runners would bring the stuff over on boats small enough to escape detection by the Coast Guard.
   
2. Industrial Alcohol - Nothing fancy here.
   Industrial ethanol (the stuff used in high school science labs) contains a denaturant, rendering it foul-smelling and undrinkable.
   A common one is methanol, or wood alcohol, which can cause the drinker to go blind or even die.
   Bigger bootleggers with a chemist on staff would buy industrial alcohol under a sham company and strip out the denaturant.
   Fly-by-night bootleggers wouldn't give so much effort and some poor souls got screwed.
   Bootleggers often added coloring or spices, slapped on a fake label and sold it as premium European scotch or gin.
   
3. Moonshine - A tradition in the Appalachians to circumvent excise taxes, moonshine is distilled corn-sugar mash.
   Moonshine production shot up in the 1920s, with many of the new cooks having no idea what they were doing.
   As a result, some of the product contained toxic compounds.
   When the moonshining industry consolidated, gangsters ran large operations and had small-time "alky cookers" producing moonshine and "bathtub gin" right in the middle of their city.
   That was the Philadelphia-based Lanzetta Brothers' main supply.
   
4. Beer - The production of O'Doul's-equivalent was allowed by law.
   Bootleggers simply spiked it to get its alcohol content back to that of normal beer, which is approximately 5%.
   
5. Wine and Cider - There was a loophole in the law that allowed the home production of fermented fruit juices, intended to not criminalize an ethnic tradition of making vinegar.
   Of course that was exploited and California vineyard owners shipped their grapes all over the country, so people made their own wine.
   
6. Sacramental wine - The amount of wine produced for Christian and Jewish sacraments shot up at least fourfold during the depression.
   There were more than a few confirmed cases of priests and rabbis producing wine for fictitious congregations or grossly inflating their congregation's sizes.
   
7. Prescribed whiskey - Easily the most amusing of the sources of liquor.
   Apparently whiskey had medicinal properties and the law provided that an acceptable use was to "afford relief to him from some known ailment.
   " This appears analogous to current medicinal marijuana laws.
   And apparently women need not apply for such qualifying ailments.
   As one could guess, doctor prescriptions of liquor increased precipitously in the 1920s.
   

People were resourceful when it came to procuring alcohol during Prohibition.
It's pretty clear in this case, that the result was a lower quality product to the people and a shift of money from the federal government (excise tax) to the bootleggers and gangsters.