A developing country is more in the category of a second world country. These countries have many of the modern conveniences, but they are not as broadly available geographically within their boundaries. Take for instance, countries that have many paved roads, but many areas that also have dirt road. Also, internet access, but the reliability is less than 100%. Electricity and water services may be present in most, but not all of the country. Reliability of these services may be poor in some cases as well.
So a second world country would be between developed countries and countries that have little or very few modern conveniences for the majority of their inhabitants.
I would place most of the Central American countries in the second world, or developing category. There has been much progress in their economies over the last 25 years, but there is still a ways to go, mostly due to the lack of true competition in their economies. But that is changing with some countries, like Costa Rica.