The display value for the most HTML elements is block, inline or none depending on the element's type.

A block element always begins on a new line and takes up the full width available (stretches out to the left and right as far as it can).

Most usefull block elements are:
<div> - a container for other HTML elements
(grouping, styling with CSS and so on... ). It provides no visual change by itself;
<h1> - <h6> - headings in a text;
(with syntactical importance from max (<h1>) to min (<h6>);
<p> - a paragraph of a text;
<form> - a form for a user input which can be sent to a server.

An inline element does not start on a new line and only takes up as much width as necessary.

Most usefull inline elements are:
<span> - is used to group inline elements in a document;
It provides no visual change by itself. provides a way to add a hook to a part of a text or a part of a document;
<a> - defines a hyperlink, which is used to link from one HTML document to another;
<img> - links image file with HTML document.