As a person is changed into a vampire, their physical appearance is enhanced, their eye colour changes, their hearing and eyesight becomes more acute, their heart ceases to beat, they no longer need to breathe. These heightened senses are invaluable to a vampire, both Edward and James, for example, rely on Bella’s scent to track her.
No further physical changes occur once someone has been transformed: "When someone becomes a vampire, it’s as if they are frozen exactly as they are in that moment." (PC1) Therefore, a vampire cannot get pregnant because this would involve a change in physical state, but if a pregnant woman were changed to become a vampire, then she would remain at that exact stage in pregnancy for all eternity.
Natural changes in the human body would not occur any longer. A woman would "no longer have any kind of ovulation cycle," (PC1), a man would no longer need to shave as hair would no longer continue to grow. Once cut, hair will never grow back, and we are given the example of Alice, whose hair is so short because "it was growing back from being shaved in the asylum." (PC1). Alice will never be able to have the long flowing locks of Rosalie.
Other things, such as fingernails, also cease to grow, but it is worth noting that a vampire’s body is like granite, so broken fingernails are unlikely! There are certainly some perks to this lack of change: Alice, Rosalie and Esme would never have to shave their legs, and none of them would have to worry about putting on weight! The human need for rest and sleep disappears in a vampire. As Edward explains, "I can’t sleep." (TW9), and "most human fluids are absent… no sweat, no tears," (PC1) and no blood of their own.
However, the venom they possess acts like a form of "saliva… the venom makes their mouths wet, at least." (PC1) Circulation ceasing and lack of heartbeat are amongst the most obvious changes when someone becomes a "cold one" (TW6). The blood that is in them when they are changed, is slowly consumed within the first year. Stephenie Meyer suggests thinking of this like using up "the fat stores in your body." (PC4), and from then onwards, the only blood that floods the old circulatory system of a vampire is the blood they have consumed. "It lightens their eyes and flushes their skin slightly." (PC1) The skin itself becomes far more refractive than a human’s skin ever can be. It literally sparkles "like thousands of tiny diamonds were embedded in the surface." (TW13). It is described as "shocking"(TW13) and also accounts for the days off school when the weather is sunny. Stephenie Meyer explains: "They sparkle because they have turned to substance that is somewhat like diamond. Their bodies have hardened, frozen into a kind of living stone. Each little cell in their skin has become a separate facet that reflects the light. These facets have a prism-like quality; they throw rainbows as they glitter." (PC7)
The body also becomes hard like granite or marble. A vampire is unlikely to be hurt or cut, simply because very little is strong enough to penetrate their flesh. We see how things that would kill a human put Edward in no physical danger at all: "The van should have smashed my legs, but you were holding it up."(TW3) Assuming something is capable of cutting through a vampire’s flesh,…there would only be blood if he/she had freshly drunk blood (and drunk a lot). Otherwise there would only be a bit of venom."(PC1)
Vampires can be killed, but humans and human tools are too weak to do it. The only sure fire way is for another vampire "to tear him to shreds, and then burn the pieces."(TW19) As far as James is concerned, "Emmett and Jasper took care of him," (TW24), so this destruction is obviously possible.