A 상가 (sangga) is a complex of shops. Many apartment complexes have a 상가 near the front or back gate. The 상가 are often so covered in signs that you wouldn’t even notice if the building underneath is made of tile or brick. At night time, the 상가 glows with the neon signs. I don’t think Korean people use a Yellow Pages directory all that much. It’s easier to walk out into the street and read the buildings until you find the business you are looking for.
I like to call the 상가 menu-buildings, since the exterior itself functions as a menu.
Have a look at these photos and try to read the signs inside.