"The airport is on the north side of the city."
why is this 北边 and not 北方? how do i know when i have to use one or the other?
方 suggests the generic type of place (not specified), whereas 边 describes the specific side. For instance,
1) 我在妈妈的旁边 I am next to my mom['s side].
2) 这是什么地方? What is this place?