Sound that is supplemental, recorded in post production, which is by its nature is understood as an effect.

Thunder, lightning, explosions, gunfire, a particularly sounding door, as well as the sound of cars.

These can also of course be entirely synthetic sounds.

These can come from a sound archive, or are a new creation that comes from the sound designer (sound editor / sound designer), or Foley Artist.

If a sound is as a "sound effect" or only a sound depends on its value within the film.

For example, the squeak of a chair can be a normal noise; if this is used to realize a scary or funny scene, it might then be considered a sound effect.