Districts of riyadh (Riyadh)

I'll be moving to |Riyadh in the next month or so.. I' trying to get an idea of the cities geography, much like any major city i imagine some districts are nicer than others.

I hear mention of northern quarter, diplomatic quarter etc... Could someone explain the differences...
Also why is it all expats are living in these compounds surely part of being overseas is living amongst the people of the city? or am i missing something?
I'm not looking for luxury just decency what areas would i find this and what prices does it come at?