In-camera or post-processing techniques for altering the look of an image.
Effects refer to techniques used to alter the look or mood of an image, for example to create a vintage or a cinematic feel. This can involve special cameras (pinhole, infrared, Lomo), lenses (tilt-shift, Lensbaby), filters (starburst, polarising), or post-processing techniques or plugins (Orton effect, Dave Hill look, Hollywood "glow", Bleach Bypass, to name a few).
If you are looking for help in obtaining a particular effect you've seen and would like to replicate, please read PSA on "What's this effect?" questions to get the best results.