Oral sex is considered a lower risk activity for STDs than are anal and vaginal sex.
Even so, it is still possible to get an STD whether you are giving or getting oral sex.
Oral Sex on a Vulva
Oral sex on a vulva is called cunnilingus. A slang term is "going down" on someone. This is a really low risk activity for HIV (the virus that causes AIDS). But cunnilingus puts you at significant risk for things that can be passed through skin-to-skin contact, like herpes.
To reduce your risk, you can use a latex barrier (aka: a dental dam) over the vulva.
Oral Sex on a Penis
Oral sex on a penis, is called fellatio or a blow job. It is a lower risk activity for HIV transmission than is vaginal or anal sex, but it is a higher risk than cunnilingus. Oral sex on a penis also puts you at risk for infections like herpes and gonorrhea.
The best way to reduce your risk is to use a condom. If you aren't using condom, avoid taking a partner's body fluids into your mouth.
Analingus, or rimming, refers to using your mouth on a partner's anus. Analingus isn't generally a risk for HIV. However, it can expose you to a number of parasites and can put you at risk for hepatitis.
You can use a dental dam over the anus to help reduce your risk.
Many people enjoy oral sex, and that is perfectly fine. Just make sure to take some safety precautions!