What Causes White Vaginal Discharge?

Hi I have been discharging a white substance 4 smetime nw I have had a baby bt that was awhile back n even b4 that arnd the time I started my period is when this begin 2 happen it has been a constant problem through out my life and I am curious as 2 what it is n why I continue 2 deal with it? It doesn't burn or itch or anything like that its just a constant everyday thing where it wets my underwear n leaves white marks n stuff on them.