What are inline and block elements?

Answer 1

Block elements are elements that can contain other elements, but inline elements can't contain any block element between their tags.
Example:
Block elements: div; p; h1; ol; etc.
Inline elements: a, b, img etc.
For more information see HTML fundamentals course, lesson HTML Basics and "Bolck and Inline elements".

Answer 2

Block elements can contains any elements, but not inlined: that's true, but not the fundamental difference ^^

+ Block element take the whole width available, and next element are placed below, on a new line.

+ Inline element takes only the needed width of their content, and next inlined element is placed on same line, without introducing a line break...