Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is the third Indiana Jones movie. We learned in the first one that Jones is afraid of snakes. This movie begins with an extended section that shows us the origins of some of Indy's trademarks - Where he got his hat, his whip, the scar on his chin, and his fear of snakes. It begins with a scene of the young Indiana Jones and a group of boyscouts on horseback stopping to explore some caves in the southwest desert. Indy and another boy climb together into one cave where they find a group of men looting it. They watch the men discover an ancient gold cross. Indy tells the other boy to go report the looters to their troop leader, but the boy feels something crawling on his lap. Indiana pulls a harmless garter snake off of him and taunts him for being afraid of it, telling him that it's only a snake. We see here that Indiana Jones was not always afraid of snakes.
As the other boy runs off, Jones steals the cross and runs out of the cave with it as the men chase him. He jumps aboard a slow-moving circus train and pushes his way through the winow of the reptile box car. He is startled by a large dangerous-looking fanged snake in a container of water, which makes him fall into a large tub full of garter snakes. the same kind of snakes he just taunted his friend for being afraid of. He finds himself covered with garter snakes and we hear him screaming loudly. This is obviously the trauma that gave him his fear of snakes.
Indy craws out of the reptiles boxcar but he feels a snake crawling under his clothing. He pulls the snake out then climbs to the roof of a boxcar where he finds the looters waiting for him. They youngest one tries to take the gold cross out of Indy's hands, but another garter snake crawls out of Indy's sleeve and onto the boy. The boy screams in fear of the snake, which lets Indy escape again, but not for long.
This brings up so many questions - why exactly would a circus ever need so many snakes? Is the House of Reptiles exhibit just piles and piles of snakes or what do they do with them? Is this the best way to transport large numbers of snakes - in open tubs with no lids, or is it the worst way imaginable? Why don't the snakes crawl away out of the tub? And what on earth is the snake in the water supposed to be and why is it just sitting in a container of water???
The snakes we see are mostly live garter snakes, but I suspect they also used some fake snakes to fatten the piles. The giant fanged snake is some kind of special effect. It may be a practical effect or maybe it's CGI. I can't tell.
Later, after the flashback when the movie becomes a hunt for the Holy Grail, we see the older Indy with some snake sculptures. One of them is a switch to turn a fireplace around.