The J-1 visa at its core is a cultural exchange program. It was created through the Fullbright-Hayes Act to foster the exchange of ideas and culture between the US and other countries. There are three broad types of J-1 visa that cover the private sector, government and academia. Together, these are made up of a total of 14 different categories. Each category has their own rules and requirements. While there is overlap between some of the categories, each one was designed for a specific purpose.
The Trainee and Intern categories are designed to allow you to gain professional training you would not be able to gain in your home country, in an industry you have prior education or professional experience in.