The term "go down" or "going down on her" etc. refers to performing oral sex. Usually it is used to mean performing oral sex on a woman but it is also commonly used to mean oral sex on a man. It just means that someone is going to take their mouth and "go down there."
When spelled: c-u-m -- it is a commonly accepted slang word for the semen or sexual liquids that come out of a person during sex, mostly when referring to men when they have an orgasm. When a man has an orgasm, he squirts or "shoots" his "c-u-m" out of his penis. This is called ejaculation. When spelled this way it just means the stuff that comes out of the penis. Not very often, but some women use this word also to talk about their own juices that comes out of their vagina.
When spelled: come -- It is the commonly accepted and quite old term referring to the actual orgasm itself in both men and women. It is the action or experience of having an orgasm.
Orgasm -- Both men and women and all animals (as far as I know) can have orgasms. One way to look at an orgasm is to say that it is the body's way of making you want to have sex. Now of course sex is fun but in practical terms it is also necessary for all creatures to procreate (have babies) or else they go extinct. So the orgasm is the incentive or reward for you taking the time to have sex. Just like a computer looks simple on the outside but you open it up and it is very complicated on the inside, so is an orgasm very complicated when you look at it with science. In very simple terms, an orgasm technically starts at the moment of any kind of sexual or romantic stimulation. This can be holding hands, kissing, talking or obviously when touching a person's penis or vagina/clitoris. If the touching or other stimulation keeps on going steady, the excitement reaches an "explosion" or peak. Just like a roller coaster. Think of it as if the sex or any other sexual stimulation is the part of the roller coaster at the beginning when you slowly roll up that first big hill. As soon as you get to the top and start rushing down really fast, that is like an orgasm and that is pretty much how orgasms work. Orgasms are pretty much the best feeling experience a person can have. It is the best feeling there is.
Yes, a person can have an orgasm even if they are still a virgin. Medically speaking a female virgin is a girl/woman who has NOT yet had a penis, finger etc. inside her vagina. Most female sexual stimulation happens outside the vagina around the clitoris. The clitoris is the pointy, soft part at the top of the inner lips (labia) about an inch above your vagina hole. So it is very easy for a girl/woman to have an orgasm and still be a virgin. Some people say that a virgin is anyone who has not had ANY sex with another person. None. No touching or anything else. If you think this is what you mean by the word virgin then I guess the only way to have an orgasm and still be a virgin is if you masturbate.