Definition
A block is an abstract model with which you define the structure of a piece of content.
Each block is made up of one or more elements.
For example, if you are creating a block for a blog article, you would create a model presumably composed of:
A textarea element for the title
A markdown element for the actual content
An image image element for the featured image
A dropdown element to create and pick categories for your articles
A Section represents the block when it has been filled with its content
When you create the block for a blogpost and add content to it, you get a section
With the term Element, we identify the fields that make up a certain block.
For instance, if the block is the model for a blog post, its elements could be a text area, media, images, link etc.