For males, masturbation -- commonly referred to as "jacking off," "jerking off," "whacking off," or "beating off" -- is the act of manually stimulating the penis. (If you've ever walked in on your older brother and saw him rubbing himself down there, or if he suddenly covered himself up, or if he was in bed and acted like you interrupted something very important, he may have been masturbating.) For older guys, masturbation usually ends with ejaculation (ee-JACK-you-LAY-shun), in which the liquid that contains sperm cells, semen (SEE-min, also called "cum," "jizz," or "spooge"), is ejected from the penis. Ejaculation -- often called "coming," "jizzing," "spooging," or "shooting your wad" -- is almost always accompanied by orgasm (OR-gaz-im), a whole-body feeling that's so powerful and pleasurable it cannot be compared to any other feeling the human body can experience.
For females, masturbation consists of rubbing the genital region and/or stimulating the vagina, which is the part of the body into which the penis goes during sexual intercourse. Women also can experience orgasm as a result of masturbation or intercourse.
If you've masturbated before, you know the answer to this question! In addition to feeling so good, masturbating relieves sexual tension, or "horniness," that builds up naturally over time. Just about every male in his teens or older gets "horny," in which his mind gets taken over more and more by sexy thoughts. This is a natural process: your sex organs produce and accumulate semen, and your body responds by causing you to want to ejaculate the semen to make room for more. One way this happens is through a "wet-dream", or nocturnal emission -- an ejaculation during sleep. Another way is through masturbation, where you stimulate the penis to "fool it" into "thinking" you're having intercourse.
It's important to note that despite what you may have heard (either through rumors from friends or from older people), masturbation is completely safe -- you will not get anyone pregnant by masturbating, and you won't catch any diseases. And it's a great way to practice for intercourse when you're ready for it and have met the right person.
If you can read this, you're old enough to masturbate! You may, though, be too young to ejaculate. Most boys begin to ejaculate semen when they're about 13 years old, but some begin to ejaculate at a later or earlier age. You can still masturbate and even have orgasms before you're old enough to ejaculate. However, the best-feeling orgasms are those that are accompanied by the ejaculation of a large quantity of semen. That's why some people like to "save it up" by not masturbating for a while.