Being a professional has nothing to do with what your job title is, it's about how you DO that job.
A slacker does what he has to to get buy, the bare minimum to keep himself from being fired. His attitude is bad, he can't be bothered with what he SHOULD be doing, just what he can get away with. He doesn't care what people think of him, or what appearance he gives. Quality of work is not his concern, pride is non-existant.
On the other hand, a professional, be he janitor, or executive, takes pride in his job. He does it to the best of his ability. He goes the extra mile to make sure his work is the best. He takes pride in his appearance, and never appears sloppy, no matter what size he is.
(side note here.. I'm one of those "big guys" and you will NEVER see me in sweats and too small t-shirt.. )
A true professional never stops learning. He keeps up to date on the latest developments in his field, so that he can better prepare himself, to keep his job done well. He is willing to help others in his field achieve THEIR personal best, without demanding anything in return, because that's the right thing to do. He doesn't try to keep down others, sabotage or insult their efforts.
In short, as I said.. it's not about what you do.. it's how you do it.. it's your attitude... it's what you are.
It's about Integrity.. doing the right thing, when nobody's watching.
Dreamer