I think the differences between of it is that developed countries refers to the sovereign state, whose economy has highly progressed and possesses great technological infrastructure, as compared to other countries than developing countries which is in process of their growth or is a country which has a slow rate of industrialization and low per capita income.
There are many factors that discriminates developed countries from developing countries. for instance per capital income, education ratio, standard of life, law and order, strong organizations and departments, democracy and many other things. In developing countries people face many financial problems and they cannot save money for their future.