A bleach is a chemical that can remove or lighten color, usually via oxidation. There are several types of bleach.
Chlorine bleach usually contains sodium hypochlorite.
Oxygen bleach contains hydrogen peroxide or a peroxide-releasing compound such as sodium perborate or sodium percarbonate. Bleaching powder is calcium hypochlorite.
Other bleaching agents include sodium persulfate, sodium perphosphate, sodium persilicate, their ammonium, potassium and lithium analogs, calcium peroxide, zinc peroxide, sodium peroxide, carbamide peroxide, chlorine dioxide, bromate, and organic peroxides (e.g., benzoyl peroxide).
While most bleaches are oxidizing agents, other processes can be used to remove color. For example, sodium dithionite is a powerful reducing agent that can be used as a bleach.
An oxidizing bleach works by breaking the chemical bonds of a chromophore (part of a molecule that has color). This changes the molecule so that it either has no color or else reflects color outside the visible spectrum.
A reducing bleach works by changing the double bonds of a chromophore into single bonds. This alters the optical properties of the molecule, making it colorless.
In addition to chemicals, energy can disrupt chemical bonds to bleach out color. For example, the high energy photons in sunlight (e.g., ultraviolet rays) can disrupt the bonds in chromophores to decolorize them.