I think you want to know how anal sex feels and how vaginal sex feels, both for you and for your partner.
At your age of 21, you must surely have some idea of how intercourse takes place. A woman has two openings – the vagina from where she pees and also has intercourse (though the internal openings are different). I have attached diagrams for your guidance.
Then there is the anus, from where waste matter is ejected. And finally there is the mouth from where she has her food.
Now, intercourse can be done in the vagina, which is normal. Then there is the anus. When intercourse is done in the anus, it is called anal sex and when the penis is placed in the mouth of a woman it is called oral sex or blowjob.
A woman can only get pregnant when you have sex in the vagina as the fallopian tubes and ovaries are in the vagina. When your sperm in the semen meets the egg ejected from the ovum, during menstruation (periods), pregnancy takes place.
So, why do people have sex in the anus (anal sex)? Because it can be more fun. The anus has sphincter muscles which can grip the penis very tightly and give more pleasure to the man. Many people enjoy this type of sex, but many do not like it at all. Anal sex can hurt if the woman is not relaxed and does not use lubricants. The anus doesn't naturally make lubrication like the vagina does, so you really need to use lube for anal sex to make it feel better and keep the condom from breaking. Don't use anything with oil in it, like Vaseline, lotion, or baby oil, which can damage condoms.
You can't get pregnant from anal sex. But there's a bigger chance of getting STDs, including HIV, from unprotected anal sex. So it's important to always use condoms AND lube during anal sex to decrease the risk of STDs.
There's also the psychological aspect of things- anal sex, because it is "labeled" as a homosexual act by the majority of the community, even when it takes place in a heterosexual context, still carries with it the connotation of "forbidden" pleasure, as opposed to vaginal sex, which is held to be "normal"