Viagra simply allows a man that has difficulty achieving and / or maintaining an erection to achieve an erect state.
It does this by opening the arteries into the penis, allowing the blood to flow.
It has no effect on women whatsoever. It has no effect on men's libido - if he is not sexually aroused, he will not get an erection no matter how many viagra he takes.
Viagra does have side effects - from affecting colour vision, headaches and so on, up to full blown heart attacks.
It is pointless for a woman to take Viagra. You can read all about how it works here:
http://www.howstuffworks.com/viagra.htm