What’s the difference between Section and Inner Section?

In Elementor to add your desired elements into template you must put them into a wrapper box called Section, each section is divided by one or multiple columns. 
To add columns to a section, read this article

Inner section

If you want to create more complex layouts in Elementor, you may need a section into a column of another section. Normally you can’t put a section into another section. In this case, you should use an Inner Section. The Inner Section allows you to create a nested section within a section.

Note: To learn more about Inner Section, please refer to this article

To be able to set equal height for columns in the section, this article will help you.