Bleach ((Eau de) Javel) contains hypochlorite or sodium hypochlorite as the active ingredient sodium hypochlorite (NaClO). It is used as a disinfectant cleaner. It can oxidize many dyes, thereby it works as a bleacher. As oxidizing agent it enhances combustion of other substances. E.g., a mixture with powdered sugar is quite explosive.

bleekwaterBy the electrolysis of an aqueous solution of sodium chloride NaCl occurs at the two electrodes two products: caustic soda NaOH and dichloromethane Cl2. Merged at lower temperatures, they form NaCl and NaClO.
In Belgium, the concentration is expressed in active chlorometric degree, in the Netherlands in the number of grams of chlorine per 100 ml. The conversion factor is 3.16. A solution of 4g/100 ml is thus equivalent to 4 x 3.16 = 12.64 degree.

A concentration of less than three degrees, is used against the formation and growth of algae in water depots and pipes.
Javel water (trade name) (8 to 35 degrees) is used as a disinfectant.
Javelextract, between 35 and 47 degrees, is used for chemical purposes.

Chlorine is a very toxic and corrosive gas that can be released when you mix bleach with other substances and products. Chloramines are chlorine - nitrogen compounds that irritate the eyes and respiratory tract.
Nowadays, you can buy bleach without chlorine. It has been replaced by hydrogen peroxide, a bleaching agent which is less damaging.