Think of a section as a photo-frame, and the blocks as the photos within.  Depending on which type of frame (section) you use, you will be able to insert difference photos (blocks) to create different content.  Each block can be persona-lised, giving you almost unlimited create license to target relevant content to your readers.

A content section can be made up of a number of content blocks

E.g. a 2-column promo area would be made of one content section and two content blocks. 

Similarly a banner carousel would be one content section with every slide being its own content block.

The screen grab below shows the different content sections making up this landing page:

Whereas the screen grab below shows the content block for the 2-column promo section:

Remember, the section level allows you to change the layout of the entire email (for example an email that starts with a banner, then a content section, then a form section OR an email that starts with a content section, then several images, then a banner). The block level allows you to change the content that shows within the sections.