Here’s an explanation so succinct that even I can understand it almost:
“Early in the formation of a galaxy or solar system, there exists a large roughly spherical swarm of small particles and molecules whizzing about the center of mass in seemingly random directions. Although there is always one direction in which more particles tend to be moving than any other direction. Particles often collide and adhere to each other, thus cancelling out much of each other’s angular momentum. Eventually, larger bodies form and almost all of their angular momentum is channeled in the direction that originally had only slight dominance. The concentrating of constituents into a flatter shape produces a gravitational influence which accelerates the process. The result is a seemingly well ordered rotating disk system as with a galaxy, solar system, or most beautifully in the ring system of Saturn for which that planet’s equatorial bulge is a contributing factor.” (source)