In optics, chromatic aberration (CA, also called achromatism, chromatic distortion, and spherochromatism) is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have different refractive indices for different wavelengths of light (the dispersion of the lens). The refractive index decreases with increasing wavelength.
To put it in simple terms; the image is moderately blurry with a fringe of two colours (usually red and blue) along the boundaries that separate dark and bright parts of the image.
It is used in artwork mainly as an artistic visual. It has gradually gained a bad reputation as an abused slap-on effect and/or a way for mediocre and/or insecure artists to essentially pad their art; being compared to lens flare, bloom, and filters.