A block contains any number of elements.
It allows for intervention in a single operation: moving, deleting, cropping, appearance, etc.
The encapsulation of one or more elements in a block is essential for certain operations: fixed width, model, selectable ...
If a block contains at least a title followed by another element, its content can be made drop-downable. At display time, the content will not be visible unless the visitor clicks the title to open it.
A block can be stretched to take full available space in its cell, and it's content can be vertically positionned.
See flexible layout to learn more.
If a block has a lower height than its container column, it can be made 'bubble' to be seen during a vertical page scrolling. The left sidebar in this page is a bubble block.
See flexible layout to learn more.