If you haven’t seen this, it’s a classic sketch with David Tennant and Catherine Tate, just before she took on the role of Donna in Season 4. I found this video of the sketch on YouTube.
Initially, Donna was a really annoying character. As the season progressed, she grew into a character who could match the doctor, be his best friend — and finally wasn’t a love interest (unlike Rose and Martha). I won’t go into the ending, but suffice it to say that the end of Season 4 isn’t very happy.