<em>
element - from W3C (HTML5 reference)YES! There is a clear difference.
The <em>
element represents stress emphasis of its contents. The level of emphasis that a particular piece of content has is given by its number of ancestor <em>
elements.
<strong> = important content
<em> = stress emphasis of its contents
The placement of emphasis changes the meaning of the sentence. The element thus forms an integral part of the content. The precise way in which emphasis is used in this way depends on the language.
The <em>
element also isnt intended to convey importance; for that
purpose, the <strong>
element is more appropriate.
The <em>
element isn't a generic "italics" element. Sometimes, text
is intended to stand out from the rest of the paragraph, as if it was
in a different mood or voice. For this, the i
element is more
appropriate.
Reference (examples): See W3C Reference