The HTML <em> element (or HTML Emphasis Element) marks text that has stress emphasis. The <em> element can be nested, with each level of nesting indicating a greater degree of emphasis.

Usage Note: 
Typically this element is displayed in italic type. However, it should not be used simply to apply italic styling; use the CSS styling for that purpose. Use the <cite> element to mark the title of a work (book, play, song, etc.); it is also typically styled with italic type, but carries different meaning. Use the <strong> element to mark text that has greater importance than surrounding text.

Content categories:
Flow content, phrasing content, palpable content.

Permitted content:
Phrasing content.

Tag omission:
None, both the starting and ending tag are mandatory.

Permitted parent elements:
Any element that accepts phrasing content.

DOM interface:
HTMLElement Up to Gecko 1.9.2 (Firefox 4) inclusive, Firefox implements the HTMLSpanElement interface for this element.

Notes:
The <em> element is often used to indicate an implicit or explicit contrast.

<i> vs. <em>
It is often confusing to new developers why there are so many elements to express emphasis on some text. <i> and <em> are perhaps one of the most common. Why use <em></em> vs <i></i>? They produce exactly the same result, right?

Not exactly. The visual result is, by default, the same - both tags render its content in italics. But the semantic meaning is different. The <em> tag represents stress emphasis of its contents, while the <i> tag represents text that is set off from the normal prose, such as the name of a movie or book, a foreign word, or when the text refers to the definition of a word instead of representing its semantic meaning.

An example for <em> could be: "Just do it already!", or: "We had to do something about it". A person or software reading the text would pronounce the words in italics with an emphasis.

An example for <i> could be: "The Queen Mary sailed last night". Here, there is no added emphasis or importance on the word "Queen Mary". It is merely indicated that the object in question is not a queen named Mary, but a ship named Queen Mary . Another example for <i> could be: "The word the is an article".
