Strictly speaking, anything that lightens the hair, bleaches it, but in the present context bleaching refers specifically to any treatment that removes colour from the hair - rather than adding colour, which is the purpose of permanent colorants. There are several different types of bleach on the market and they range from the mild brighteners that lift hair colour a couple of shades, to the more powerful mixes that completely strip hair of its natural colour.
Hair Bleaching is quite difficult process to do and is best left to a professional hairdresser. If misused it can be very harsh and drying on the hair.
To get the best results, make sure that your hair is in optimum condition prior to bleaching. Once the hair has been bleached, regular intensive conditioning treatments are essential.