Use the adjective developing to describe a country that's relatively poor and has a lower standard of living than most other countries in the world.
There's not a specific standard for calling a country developing, but there are some societies that most of the world agrees are less industrialized than most. Developing countries generally have poorer people and a less vibrant economy than more developed countries. Some countries object to being called developing, since it implies that all countries want to become "developed," or more Western in their economy and government, which is not necessarily true.