"Very smart owl you've got there. Arrived about five minutes after you did. If there's anything you need, Mr. Potter, don't hesitate to ask."
-- Tom the innkeeper of the Leaky Cauldron (PA3)
Hedwig is Harry Potter’s snowy owl, his companion and post owl. Hagrid bought Hedwig as an eleventh birthday present on Harry’s first visit to Diagon Alley (PS5). After returning to the Dursley’s, Harry found the name “Hedwig” in the book A History of Magic and borrowed it for his owl (PS6).
Companion to Harry Potter
Loving, loyal, and stubborn companion to Harry Potter
Hedwig's name was taken from St. Hedwig, who was a Medieval saint (JKR). This was probably St. Hedwig of Andechs (1174-1243) who was a Polish duchess and is the patron saint of brides, death of children, difficult marriages and widows (Catholic Encyclopedia).
The death of Hedwig symbolizes a loss of innocence for Harry Potter; just following the battle where Hedwig dies, Harry turns 17, the adult wizard age.
Naturally I gave my hero what I consider to be the most beautiful owl of the lot: the Snowy Owl, which also goes by the name of Ghost Owl. These are not native to Britain, so I felt that she would give Harry kudos at Hogwarts (there is no other snowy owl there, as I trust you have noticed). However, any owl expert would tell you that Hedwig is strangely atypical of her breed. Only after Philosopher’s Stone had been accepted for publication did I realise that Snowy Owls are diurnal. I think it was during the writing of Chamber of Secrets that I discovered that Snowy Owls are also virtually silent, the females being even quieter than the males. So all of Hedwig's night-time jaunts and her many reproving hoots may be taken as signs of her great magical ability or my pitiful lack of research, whichever you prefer (JKR).
Hedwig is played by Gizmo in the films, with stunt replacements, Ook and Sprout.
The Snowy Owl (Bubo scandiacus) is not common in the British Isles. They are found mostly in Canada, and other norther parts of the world.