"An attitude" is a usage that has been around for some time. To say
someone "has an attitude" is often an expression of disapproval,
usually said about someone who is being high-handed, tempermental,
uncooperative, superior, and/or just plain difficult.(See also:
"Striking an attitude", which according to Webster's New World
Dictionary, Third College Edition, means "to assume a posture or pose,
often an affected or theatrical one.")
But, depending on the context nowadays, possession of an attitude can
be good. Take this website for example, created by a sports fan who
wanted a place where people could freely express their opinions:
www.sportsrant.com. "Sports talk with an attitude"
Attitude here seems to mean no holding back, no matter how negative or
controversial the opinion may be.
In-your face self-assertion and confidence, without inhibition, is
what having attitude is all about, in the contemporary sense. And
since a picture is worth a thousand words, I refer you to this image
of "attitude" (or "tude" in slang shorthand):
The little girl in this photo is expressing her personhood in no
uncertain terms...no camera-shyness here, no inhibition. The photo's
title: "She's got attitude."
I hope this helps.