The sewers are below the streets of Slagtown, where deformed chuds and nicotine addicted chobos go around beating people to smith and takin' their lighters. Be careful, if you venture too far down, you may come across a beast known as the Charnold, the Kraken, or the dangerous bug queen who could and possibly will consume you. The three of which, when you are newer, can massacre you. But, later on, the sewers will be a fun and mildly challenging place to train your skills and collect some chudskins.
A detailed map with descriptions can be found here.