Why does my washing machine smell?

it seems it may have mildew in it? the clothes seem to smell musty even after they just finished being washed. i leave the door to the washer open b/c i know it helps it air out. but i think somehow i got mildew in it? i don't know where mildew accumulates in washers. why do you think this is, or what is it and how do i get rid of it? thanks for your help.