Ordet byer dækker over alt fra relativt små byer til millionbyer, fra traditionelle servicebaserede økonomier til industrielle centre, fra centre i næsten tomme regioner til dele af kæmpestore, tætbefolkede byområder.
Cities vary from relatively small to clusters with millions of inhabitants, from traditional service-based economies to industrial centres, from centres in almost empty regions to parts of huge and densely-populated urban areas.