The individual units normally share walls, but that isn't a requirement. The main difference in condos and regular single homes is that there is no individual ownership of a plot of land. All the land in the condominium project is owned in common by all the homeowners.
Usually, the exterior maintenace is paid for out of homeowner dues collected and managed under strict rules. The exterior walls and roof are insured by the condominium association, while all interior walls and items are insured by the homeowner.