The video is short — just 19 seconds — and not particularly compelling. A viewer would be forgiven for clicking away before it ends.

The grainy footage, uploaded on April 23, 2005, of a man standing in front of the elephant enclosure at the San Diego Zoo — “All right, so here we are in front of the elephants” — does not look like the sort of thing that would touch off a video revolution.

And yet, two decades after that inauspicious start, YouTube is now a cornerstone of the media ecosystem. It’s where people go for music videos and four-hour-long hotel reviews. It is a platform for rising stars and conspiracy theorists. It’s a repository for vintage commercials and 10 hours of ambient noise. It has disrupted traditional television and given rise to a world of video creators who make content catering to every imaginable niche interest.

For every YouTube video you have watched, there are hundreds of millions you will never see.

Here’s a look back at some of the biggest moments in YouTube history.

April 23, 2005

YouTube Begins

The first video uploaded on YouTube is titled “Me at the zoo,” and it shows one of the platform’s founders, Jawed Karim, admiring the elephants at the San Diego Zoo. Mr. Karim founded YouTube in February 2005 with Chad Hurley and Steve Chen, but left the venture in 2006.