This tag adds an underline effect to the text within.
<u> This is some underlined text </u>
Would show up like so:
This is some underlined text
Note the fact that the underlined text looks similar to a hyperlink. This is especially true if the text is the standard blue color.
The command that will produce this same effect in CSS is text-decoration.
<u style="text-decoration:none;"> This is some text in an underline tag </u>
would show up like so:
This is some text in an underline tag
The "text-decoration:none;" completely negagates the styling of the <u> tag.