The Disc

Why are all the planets arranged in a (more-or-less) flat disc around the Sun?

It has to do with the formation of the Solar System.

The matter spinning around the center (which was to become the Sun) was pulled towards the center because of the centrifugal force and gravity. The free flying objects crashed and collided while spinning, forming planets, and the rest became asteroids, meteoroids, comets, and dwarf planets. The aligning of the materials around the Sun resembles the rings around the Outer planets.

The Earth isn't flat, but the Solar System is!

Some objects in the Solar System do not rotate around the Sun in the same plane (level) as the others that are in the 'disc'. Do you know which ones?