This ancient and mysterious charm conjures a magical guardian, a projection of all your most positive feelings. The Patronus Charm is difficult, and many witches and wizards are unable to produce a full, corporeal Patronus, a guardian which generally takes the shape of the animal with whom they share the deepest affinity. You may suspect, but you will never truly know what form your Patronus will take until you succeed in conjuring it.
MIRANDA GOSHAWK'S OVERVIEW OF THE PATRONUS CHARM
The Patronus Charm (Expecto Patronum) was the most famous and one of the most powerful defensive charms known to wizardkind.[4] It was an immensely complicated and an extremely difficult spell, that evoked a partially-tangible positive energy force known as a Patronus (pl. Patronuses)[5] or spirit guardian.[3] It was the primary protection against Dementors and Lethifolds, against which there were no other defence.
There are two types of Patronuses: corporeal which means a Patronus with a particular shape and form and incorporeal Patronus. Incorporeal Patronuses have no particular shape and do not protect against dementors the way corporeal Patronuses do. However, some wizards like Remus Lupin choose to cast incorporeal Patronuses to hide their identity.[1]