Very, very few artists can be said to have changed the course of their medium even once. Miles Davis changed the direction of jazz three times.
First with 1949's The Birth of the Cool, Davis, early in his career as a bandleader, slowed the frantic tempo of bebop down, and introduced the world to cool jazz. This would be the dominant form of jazz, especially as played by west coast musicians, for the next decade.
In 1969, Davis released Bitches Brew, a double album of what would eventually be described as jazz-rock fusion. Fusion of course, would be the dominant form of jazz (for better or worse) for the next decade, and the players on Bitches Brew (which include John McLaughlin, Joe Zawinul, Wayne Shorter, and Chick Corea) would be its chief proponents.
In between those two extremes, in 1959, Davis introduced modal jazz to the world. Modes are scales of musical notes, some of which date back thousands of years. The appeal of modes for jazz musicians was to get away from playing constant chord changes, which were felt to hamstring the soloist, and provide a sparse, less cluttered background for solos, providing maximum flexibility and expressiveness. In the hands of an amateur, who needs the chord changes to influence his selection of notes when soloing, the result is clichéd scale after scale (this would become the curse of the fusion genre). But in the hands of master musicians such as Davis and his bands, the results were obvious: a new composition style instead of relying of old standards, a greater freedom of expression, and a unique new sound that would be jazz's dominant form for the next 30 years, and remains one of its most important elements. The album that introduced this sea change was of course, 1960's Kind of Blue.
Along with John Coltrane's A Love Supreme and Giant Steps, Kind of Blue is one of those albums that even non-jazz fans own--they are definitive recordings from the 1960s. And yet, no album emerges in a vacuum. There's rarely a moment of divine inspiration behind an artwork-it's almost always a combination of talent and hard work, combined with an enormous amount of thought.