Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a classic slasher movie, the first of many in the Friday the 13th franchise. It has been criticized for being misogynistic, but unlike a lot of slasher movies, this one kills men with as much enthusiastic violence as it kills women, and the killer actually turns out to be a woman. The hero who finally unmasks and kills her is also a woman. The victims are a group of young attractive camp counselors who smoke weed, have pre-marital sex, play strip monopoly, and, worst of all, they kill a harmless snake. As payback for that and the rest of their sinful behavior all but one of them gets either stabbed, axed, impaled, shot to death with arrows, or has their throat slit with a knife. The human killings are all faked with prosthetics and makeup, of course, but the snake was really and cruelly and sadistically hacked to death with a machete.
According to Reptiles Magazine (June 13, 2014), the snake was not in the original script. The visual effects man accidentally discovered a snake in one of the cabins and decided to add a snake-killing scene. As far as I can they used a black ratsnake, which would be found naturally at the filming location (a real camp in the woods in New Jersey.) The Reptiles article questions whether the snake is real, but it definitely is, and it is alive before it gets hacked up. You can see it moving before and after. The web is full of other comments about the snake scene, including a story that the snake was someone's pet and that the owner didn't know the snake was going to be killed and you can hear the owner crying in the background. (I thought the women's voices we hear came from the actresses.)
The snake scene:
Alice (Adrienne King) is alone in a cabin wearing a pink bathrobe and opening some dresser drawers when she sees a black snake at her feet. She screams in fear and the snake crawls under the dresser. She runs to the door to call for help and Bill (Harry Crosby) who was cutting weeds with a machete runs inside. She tells him there's a snake in the cabin. He says "What do I do?" She tells him to kill the snake. He's afraid it might bite him. Then the others come inside and several of them look under the bed. Alice tells Bill to call the snake. He asks how do you call a snake? Jack (Kevin Bacon) screams that he will flush it out and jumps on the bed. That works. The snake crawls into the open. People throw things at it and try to hit it with pillows. Then we see Bill hack the snake two times with the machete. They start making jokes and one of the women named Marcie says "At least we know what's for dinner." Everybody groans and another woman hits her with a pillow. Unfortunately, nobody gets a chance to eat the snake, because they all get hacked up later themselves.
Whether or not the accidental insertion of the snake scene is a true story, the scene works because it's a classic horror movie example of foreshadowing and mis-direction - we see something frightening and violent that has nothing to do with the violence to come, and when it's over, and everybody is relieved, we also feel relieved. We're caught off guard, but also prepared when people start getting hacked up the way the snake was.