The viewport is the window or viewing area that displays web pages. When the viewport is smaller than the web page, scroll bars should be available.
The initial containing block is the entire width and height of your web page - including parts of the page that are outside the viewpoint.
A containing block is a box or block that contains other elements (descendant boxes). An element's containing block means "the containing block in which the element lives".
Block level elements (or block boxes) are elements that are formatted visually as blocks.
Inline elements are elements that do not form new blocks of content; the content is distributed in lines.
Normal flow is the way a document will display if you had no positioning or floating applied to elements. The content will flow down the page, starting with the first element in your document and finishing with the last element in your document.
When a box is taken out of normal flow, all content that is still within normal flow will ignore it completely and not make space for it.