Broadly speaking, Sound Effects (SFX for short) refer to elements such as gunshots, explosions, sci-fi sounds, and the like and Foley for man-made noises, such as clothes rustling, doors opening, footsteps, etc.
Foley (besides as a special effect) should not be "heard separately" but instead perceived unconsciously by the viewer. SFX on the other hand are an important ingredient in credibility, although they should not be overdone lest it distract attention.
With help of huge libraries and/or specially recorded ambient noises, your production can be enhanced or supplemented. I also work with sound designers and foley artists who specialize in live action or animation.