The Sewers are a side entrance to the Undercity, accessible from the west of the main gate to the Ruins of Lordaeron. This is often the preferred entrance to the city by Alliance who wish to attack the city, bypassing the main entrance.
Likely due to the nature of the area, this is the only part of Undercity to have retained it's Abomination guards.
The Canals are the areas between the four quarters of the Undercity. A river of green "liquid" flows in a large channel around the quarters in a circular path.