The Patronus Charm (Expecto Patronum) is the most famous and one of the most powerful defensive charms known to wizardkind. It is an immensely complicated and extremely difficult spell that evokes a partially-tangible positive energy force known as a Patronus or spirit guardian. It is the primary protection against Dementors and Lethifolds, against which there are no other defence.
To successfully cast the spell, one must muster the happiest memory they can think of (the happier the memory, the better the charm will work) and begin drawing circles with their wand so as to increase the power of their spell. They must then say the incantation, Expecto Patronum; the Patronus will come from the tip of the wand and can be directed towards a target by pointing one's wand at said target. It is possible to disguise the form one's Patronus takes, which was done both by Severus Snape to hide his love for Lily Evans, and by Remus Lupin, who felt it would give away his condition.