A few questions from an embarrassed 21 year-old

Erection is when the penis is stiffened and raises up were it's filled with blood when a man feels sexually aroused/or ready to have sexual intercourse same with women with their clitoris but the clitoris doesn't insert into anything just when women feel aroused.

Coitus is when the man's penis is inserted into the woman's vagina for sexual intercourse.

While having sex, a man and a woman feel pleasure and moan for most couples and then the man ejaculates in the woman for conception. unless they use contraceptive protection like birth control pill (for woman to prevent ovulation), they also can use IUD that is placed in the woman's uterus, or condoms, diaphragm, and even foam/jelly supply to kill sperm.
But anyway, as both couples have sex, they feel intense pleasure and ejaculation occurs and they sweat as the man is thrusting his penis in the woman as it can last for hours depending on some couples.
Sex can also relieve stress.
As we all know that's how babies come from.

an erection occurs when the male gets sexually aroused, causing the penis to harden to prepare for when he y'know, sticks it in the designated location of the woman. If not for this, it's like trying to shoot pool with a rope I/ll tell you that.
coitus just means the act of sex (erection going inside the lady parts along with the thrusting movement to add friction for the pleasure of both participants)
The basic point of this act is to make your partner reach an orgasm, which is like an explosion of pleasure within your loins and that's how babies are made but it is also how sexual diseases are spread, so be sure to use a condom when you do have sex. I hope this has been helpful for you and i am willing to clear up anything i may have confused you about in this description.
-Elixxe14