Planetary migration is a change in a planet's orbit, typically because of interaction with a disk (planet-disk interaction, gas disk migration or just disk migration) or planetesimals. This sort of gradual migration is called smooth migration, as opposed to violent migration due to close encounters or collisions of planets (planet-planet scattering).
Planetary migration due to interaction with gas disks is classified by the underlying mechanism as follows:
Type I migration is due to torque created by interaction with spiral density waves the planet creates in a disk. Planets of terrestrial mass can be moved this way. Timescale is on the order of 100 million years to migrate from 1 AU into the star so it is almost too efficient.
Type II migration is due to interaction with material filling a disk gap formed by the planet. Planets with more than 10 Earth masses can move this way and it is a way hot Jupiter can be positioned.
Type III migration is due to interaction with large-scale vortices within the disk.
Planet-planet scattering (gravitational scattering) seems necessary to explain observed orbital inclinations, which include misalignment between planetary orbits and stellar rotation, and even retrograde orbits. Smaller inclinations might be ascribed to a tilt of the star or a tilt of the disk by some unknown means. In simulations, planet scattering has produced larger inclinations, including a coplanar flip.