The story isn't long, but it does seem important. It has continued to be represented in art and in story (though there have DEFINITELY been some changes made to make it fit into the new cultures), but it was also super important in ancient Greece, where sacrifices were made in a festival in his honor. Rituals were done in his name. I will probably forget to follow up on this and read more articles about why, so if you know, you should email me!
Anyway, a short summary is made by Apollodorus (1.9.19), which I will translate myself (since I don't like Frazer's translation at all here):
"For Hylas, the child of Theiodamantos, and the lover/beloved of Herakles, having been sent off to fetch water was snatched* by the nymphs because of his beauty."
This story is found slightly more fully in Book One of the Argonautica, and in that version, it is clarified that he walked off from the group hoping to get water to make a meal for his man. His man, of course, was Herakles. Now, we should sidenote here that Herakles definitely liked a sexy youth or two. Hylas is the most famous of his beloveds, but by no means the only one. This does not mean that Heracles was gay. It does not even mean that he was bi. These terms did not even exist. Although I think this topic is fascinating, I will suffice it to say that the relationship between Hylas and Herakles was very normal and even celebrated in ancient Greek culture. So it would make sense that Hylas (the beloved and "passive" partner) would be getting water to make something nice before Herakles got back to camp. But, unfortunately for him, he was deep in Pegae territory. Now, most of them were away guarding the forest for a nymphaic jamboree they were planning on throwing for Artemis, but one chick was left to hold down the fort, I suppose and when she saw Hylas, standing there in the moonlight (moonlight is like mood lighting ...) lookin' all pretty and sexy, her heart just went pitter pat. It didn't help that Aphrodite was there aiding the whole process (didn't help Hylas that is). And if you have read about the nymphs, then you know, this is NOT a good situation our young man is walking into. The authors have already established that he's kinda delicate, that he's relatively passive in his relationships, he's young, and he's mortal. Said nymph may appear delicate, but she is super powerful compared to this dude, and no good can come from relationships where women hold more power than men - at least not in ancient Greek mythology.
You already know what happens next, but I'll spin in out all poetic like. Hylas, dipping like a dancer, kneels and drags his pitcher through the water. And as his arm went into the water, her arm came out and wrapped around his neck, pulling him closer for a kiss. Truly, our source must have been a mouse watching from land, because the story ends there. We do not even know if she got her kiss, because when Hylas tumbled into the water, it was the last anyone ever heard of him. Sure, Herakles searched the island for a long time, but eventually, the crew of the Argo gave up and continued their quest without him.
If you are like me, you will want to know more about gender and sexual relationships now. For the meantime, I can only direct you to my own essays on the topic. Enjoy!
*It is worth noting, I believe, that the word used for "snatched" is "harpazo" - the same word that's used to form the word Harpy. Although it is more common to see things like centaurs carrying off virginal young women, it is not uncommon to imagine women with this imagery of snatching and stealing.