What are the differences from a featured image and thumbnail, and the reason to use one within the other?

What are the differences from a featured image and thumbnail...?

The title! 'Featured images' in 3. are what 'post thumbnails' were by 50 percent.9.

The code is identical, but from the consumer experience position, I believe they made the decision that 'post thumbnail' would be a little ambiguous and/or confusing.