Bleaching is the only way to lighten hair significantly -- you can't just dye it blonde. When you bleach your hair, hydrogen peroxide removes some or most of your natural pigment from the cortex; this is the first step of the double process. The second step is toning -- a rinse is used to shade your newly blonde hair a warm (reddish), cool (ashy) or neutral hue. Bleaching is permanent and damaging, and you'll have to touch up your roots every four to six weeks -- sometimes a tricky thing to do on your own -- so think carefully before taking this big step. Unless you're already blonde, it's best to let a professional colorist handle the task at least the first few times. If you're doing it yourself, a strand test is essential. (For blondes who just want to go one or two shades lighter at home, there are one-step products on the market that combine a low level of peroxide with a toner.)
The active ingredient in the bleaching process, peroxide, comes in varying strengths, referred to as "volume." Twenty- and three-volume are used for most hair; ten-volume peroxide lightens hair by about one shade, twenty-volume by two shades, and so on. Rarely is peroxide over forty-volume used.
Blonde is the usual result women want from bleaching, but it's not the only one. If you want to permanently dye your hair from red or blonde to a very dark shade, or from any color to something wild like magenta or blue, you'll have to bleach your hair first so that the new color can take hold.