All of the text within the <blockquote> and </blockquote> tags is set off from the regular document text, usually with indented left and right margins, and sometimes in italicized typeface. Actual rendering varies from browser to browser, of course.

The HTML standard allows any and all markup within the <blockquote>, although some physical and content-based styles may conflict with the font used by the browser for the block quote. Experimentation will reveal those little warts.

The <blockquote> tag is often used to set off long quotations from other sources:

We acted incorrectly in arbitrarily changing the Kumquat
Festival date. Quoting from the Kumquat Growers' Bylaws:
<blockquote>
  The date of the Kumquat Festival may only be changed by
  a two-thirds vote of the General Membership, provided
  that a <strong>60 percent quorum</strong> of the Membership
  is present.
</blockquote>
(Emphasis mine) Since such a quorum was not present, the
vote is invalid.