Once, she referred to Dumbledore as an "obsolete dingbat". When she wrote about the Dark Mark appearing at the Quidditch World Cup, she said that there were rumours that numerous bodies had been removed from the forest (thus creating a rumour which had not existed before, and was blatantly untrue). She reported on Ludo Bagman's trial (many years previous to when the story takes place) when he was accused of being a Death Eater.
Harry first encountered Rita when she was planning to write an article about the Triwizard Tournament, in Harry Potter and the Goblet of Fire. He didn't give her the kind of answers she wanted in the interview (which took place, oddly enough, in a broom cupboard). When they took pictures for the article she asked for separate pictures of all the champions. She then secretly interviewed several people about Harry.
Her article about the tournament turned out to be mostly about Harry. A picture of Harry's face took up the front page, and she replaced Harry's answer in her interview with those of her own invention, making it seem that he wanted attention. She stretched facts to make Harry seem like a tragic hero. The other Triwizard champions' names were squeezed in at the end of the article, and Cedric Diggory (the other Hogwarts champion) wasn't mentioned at all.
Her article made life very hard for Harry, and Dumbledore banned her from the grounds. Rita interviewed Hagrid, but rather than talking about creatures she just wanted information about Harry. Hagrid commented after the interview that she seemed disappointed when he said that he'd never had to tell Harry off.
Rita Skeeter was, in fact, an unregistered Animagus who could turn into a beetle. She decided to get revenge on Hagrid by coming to Hogwarts during the Yule Ball, and spying on him while transformed into a beetle. She overheard Hagrid tell Madame Maxime that he was half-giant (which made him extremely unpopular, due to the well-documented ferocious nature of giants), and printed a story about it. With the addition of Malfoy's hippogriff incident, she made it seem as though Hagrid were dangerous; so much so that Hagrid became severely depressed and nearly resigned.
When she encountered Harry, Ron and Hermione in Hogsmeade, Hermione insulted her; and, predictably, she wrote a horrible story about Hermione. As she was one of Harry's best friends, and as she had a relationship with Viktor Krum, Rita portrayed her as a double-crosser, which made life hard for Harry, Hermione and Viktor Krum. Her last defaming article stated that Harry was "disturbed and dangerous", using comments made about him by Draco Malfoy and Malfoy's cronies. She also cited Harry's ability to speak Parseltongue (the language of snakes), and pains in the scar given to him by Voldemort, as additional material for her article.
Since Rita had defamed her, Hermione had been bent on finding out how Rita was spying on private conversations. She found out, and trapped her (as a beetle) in a jar, just as Dumbledore was reforming the Order of the Phoenix.
Hermione made Rita swear to "keep her quill to herself for a full year" or she would tell the authorties she was an unregistered Animagus; Rita became unemployed. About a year later (in Harry Potter and the Order of the Phoenix, Hermione forced her to interview Harry about Lord Voldemort returning and to submit the story to The Quibbler. Until then, almost nobody in the general wizarding public believed that Voldemort had risen again. The article helped Harry and his friends greatly; particularly because Dolores Umbridge banned it from Hogwarts, in an attempt to stop the information in the article from spreading. But this attempt backfired; because all the students at Hogwarts became desperate to read it, to see why it was banned!