Prisoner of Azkaban, Professor Lupin described the Patronus as, "a kind of anti-Dementor - a guardian which acts as a shield between you and the Dementor."
Harry's Patronus has gotten him out of quite a bit of trouble over the years and the spell can only be used by the most skilful of wizards. Harry's Patronus comes in the form of a Stag and was unleashed when he uttered the spell, "EXPECTO PATRONUM!".
The Partronum took the same shape as his father's. He first sees this in a beautiful scene in which the stag strolls out onto the waters of The Great Lake with Sirius Black and Hermione.
The Harry Potter Harry's Patronus Pop! Vinyl Figure comes packaged in a window display box.