In book one, she’s described as being a snowy or white owl.
Do we know what species she is in either the book or the movie?
"White owl" is just a description of her, but "snowy owl" is actually a species of owl, native to the Arctic. It is the only species of owl that is predominantly white with black markings, and therefore unmistakable. Here's a quote to confirm that this is Hedwig's species:
Harry now carried a large cage that held a beautiful snowy owl, fast asleep with her head under her wing.
-- Harry Potter and the Philosopher's Stone
Here are two pictures of snowy owls (female on the left, male on the right):
In the films, Hedwig is played by male snowy owls, which are whiter and also smaller, making them easier for young actors to handle. You can see this from comparing the follow shot of film-Hedwig to the two pictures above: