It is called oral sex (cunnilingus) and it is more than just licking. It is also some GENTLE sucking and nibbling. It is one of the easiest ways for women to experience an orgasm.
Talk to he and find out if she wants you to. Once you do it, it is also advisable for her to guide your head and talk to you to tell you where and how hard and fast to lick. If she is giving you oral sex, it is god for you to return the favor.
The risks are of infections. If you have herpes on your lips (cold sores), stay away from her as it can jump to her genitals. If she have some sexually transmitted disease (STD) or sexually transmitted infection (STI) you can contract it. If you have any doubts about her having sex with other men and contracting some STD/STI, you should use a dental dam over her vulva before pleasuring her. You can make your own dental dam by cutting a condom open and placing it between your mouth and her vulva. It may also be a good idea for her to be tested for STDs/STIs if she has been sexually active before.
Another thing you should never do is blow air into her vagina. It is a strictly licking/sucking/nibbling technique - NO BLOWING.