You all know a patronus, right?
(HINT: It's that thing in Harry Potter ...)
In case you don't, this is a patronus:
This is a patronus, according to the Harry Potter Wiki:
The Patronus Charm (Expecto Patronum) is an immensely complicated, very difficult spell that evokes a partially-tangible positive energy force known as a Patronus (pl. Patronuses) or spirit guardian. The charm was primarily designed for defence against Dementors and Lethifolds, against which there is no other protection. Albus Dumbledore, however, invented a means of using one's Patronus as a messenger to other witches and wizards, and this is the method of communication that the Order of the Phoenix uses.