Quoting NICKEL☮POLIS:" Is Placenta Previa just a random occurance? It's when your placenta is where... covering the cervix? ... [snip!] ... the cervix? Do you need a c section because of it? Sorry I'm so ignorant I've never known anyone personally who has had it."
It's when baby implants close to the cervix and the placenta grows close to or covers the cervix. If it doesn't 'move' (it doesn't really move, just as the uterus stretches and grows you are able to tell just how close it really is) then you need a c-section. It can cause bleeding during pregnancy, as any dilation will disturb the placenta and can cause abruption. From my experience, they like to have 2cm of clearance before they will ok you for vaginal birth.