T'Pol is a young, emotionally confused Vulcan who has no real experience in her own culture's history or the galaxy at large. She has jilted her fiance and betrayed her government because the humans on Enterprise make her feel important. Her only true assets are on her chest.
She believes that demeaning and insulting people is prudent diplomatic behavior. She once derided a human by calling him an "impulsive carnivore" after he ate a breadstick. Later she showed a massive lack of concern for her fellow crewmates when she used narcotic minerals (to Vulcans, at least) intended to help shield the ship to get high. These were the same materials that the crew weren't using because of their concern for her well being.
She was the first officer and science officer of the USS Enterprise.
Behind the scenes
Almost everything about the character was the result of the creators of the show attempting to engineer a breakout character from the beginning of the series (just like they tried with Neelix before her, you'd think they'd learn). They took the attributes of one of the two breakout characters from Voyager, Seven of Nine, and generalized them to create T'Pol. This of course included looking for an actress based on her figure rather then her acting ability. In the case of Seven, they got lucky, as Jeri Ryan was capable of making more of her character than the writers intended (as with the Doctor before her).
Like with Neelix, this did not work out well.