Katniss is a normal child in District 12, the coal-mining District; she is smaller than people of her same age from other areas because of the very poor conditions she lives in. While she is not excessively short, she is very malnourished because of the difficulty in getting food; as she fills out in the Capitol prior to the Games, she becomes stronger and more conventionally attractive.
Katniss describes herself early in the book, in relation to Gale, her good friend:
I watch as Gale pulls out his knife and slices the bread. He could be my brother. Straight black hair, olive skin, we even have the same gray eyes.
(Collins, The Hunger Games, Google Books)
Her hair is longer, and Gale is taller, but their physical resemblance is close enough that they pass for cousins in the second book. Katniss doesn't have the cultivated beauty of the Career tributes, who spend their time training to win, but she has a personal magnetism that attracts both the watching crowds and the hidden rebellion.
Katniss has straight long black hair, which she normally pulls back into a long braid, beautiful olive skin, and gray eyes. She is small in stature and light for her age, being one of the smallest tributes in her games, but due to her outdoor life, she is strong for her size. She is also known as "the girl on fire" during the Hunger Games; she earns this title from her fierce personality and the outfits created for her by her stylist, Cinna.
In the first book, her hearing in one ear is destroyed because of an explosion caused in the arena, but it is fixed in the Capitol in the end, as are all of her other major and more minor injuries including the erasing of any scars.