There is an element of flabby skin for some. Skin's natural elasticity makes it so that the skin isn't nearly as stretched as it was when pregnant, but it does stretch. Stretch marks are evidence of that as well.
I asked my mother the same thing once when I was about 14. She said that after women give birth, they usually go through a period where they bleed for a time. It's similar to menstruation, but the fluid is a bit different and varies depending on how you give birth. For a few weeks afterward there is often cramping as well. These things happen because the uterus is returning to its normal size (or close to it).
Of course the bleeding or discharge after birth and the effects of pregnancy on the skin depend entirely on each woman and each pregnancy and can vary hugely.
I found this forum which has a lot of answers to that question and similar ones:
http://www.babycentre.co.uk/bbs/552678/. Watch out if you're easily grossed out, though, because they are talking about kind of "yucky" things. The same website also has a section of articles on the topic:
http://www.babycentre.co.uk/postpartumhealth.
Now I've never been pregnant, so I'm certainly no expert. Are you actually pregnant or just inquisitive?