Abstinence means choosing not to have intercourse. There are all kinds of ways to accomplish this-- you may abstain from all sexual contact, or you may kiss and pet and even have oral sex. The important point is that there is no penis-to-vagina contact. None. The man secretes sperm-laden fluid long before he ejaculates, which can result in pregnancy as well as transmission of STD's. Don't push your luck on this point.
If abstinence is your only form of protection, it must be practiced scrupulously. However, the reward for doing so is 100% reliability. It's an all-or-nothing deal which many women are quite satisfied with.
One word of caution. If you choose to engage in sexual contact without intercourse, you are sharing the responsibility of protection with your partner to a greater degree than with other methods. Once you get the man to put on the condom, or to wait until you get your diaphragm in, or whatever, the precaution has been taken. But with abstinence, you must constantly be aware of what actions could be dangerous, and you are partly relying on him to remember also. Abstinence is a more intimate form of protection than most other methods. I recommend practicing it only with someone you really trust, or else practicing it to the extreme (ie, no sexual contact).
Abstinence should not be confused with a technique called "withdrawal," in which the man does penetrate, but pulls out before ejaculation. Withdrawal does not work! The pre-ejaculate has plenty of sperm in it, and can transmit STD's (including HIV). Not to mention that it's quite difficult to stop oneself just before an orgasm and remember to pull out; many men don't accomplish the withdrawal. Don't get bullied into using this method if what you want is abstinence. That's why you should only attempt sex-without-intercourse with a truly trusted partner.
If you are practicing complete abstinence from sexual contact, you may nontheless want some sexual release. (Or you may not feel this need; either situation is normal). I recommend masturbation. You can do it with just…