a patronus is a protective spell. it's basically casting a spell that will go out and fight for you. it tends to take the shape of a spirit animal and be tied to your personal history. ie harry's is his father as a horse (his father was an animagus who could turn into a horse).
█▆▇▅▇█▇▆▄▁▃ Big PEMFin H & z's "I ain't no entertainer, and ain't trying to be one. I am 1 thing, a musician." � Miles
"When the music stops he falls back in the abyss."