| A large canister with a hose connected to the side. On the top of the canister is fitted a powerful fan that sucks air out of the canister through a filter. This produces a vacuum inside the canister. Air rushes in through the hose to fill this vacuum, and carries all types of dirt, crap, even liquids with it.
The nice thing about a shop vac is that if you accidentally suck up something important, it is easy to open the canister to retrieve it. This is in contrast to a regular vacuum cleaner, in which you have to poke through the messy bag and make a huge mess.
Being able to suck in liquids without mucking up the motor is great too. Unfortunately, shop vacs are tougher to clean out than regular vacuums; with regular vacuum cleaners you just toss the bag. With a shop vac, you gotta empty the canister into a trash receptacle, producing an impressive cloud of potentially toxic dust. | 
