What I think they are
Startups are hard. That's a fact, not an opinion (everything from now on is my opinion).
In a broad sense, startups are tech enhanced companies. They grow and fail fast.
You've probably heard of a couple companies that started as startups such as Apple and Microsoft, but it comes a moment in the lifecycle when they become scaleups, and then ful fledged companies, but that's for another post.
I've worked in the most iconic startup in Colombia: Rappi. I stumbled upon it and hacked my way into it. I was employee 52 and left after a year, when there were more than a thousand. It was exhilarating. Every week felt weird, at the end there were so many changes, everything was hectic. Planning happened in the morning and deployment of such plans happened in the afternoon. It all zoomed.
This is the usual idea in startups, where speed is everything. Money is short and you need to prove yourself. Some raise money from investors, some bootstrap, but all are playing David v. Goliath.
Startups love speed, and that creates a lot of holes to plug later o. Everyone wears many hats, one day I was in marketing and the next with a distributed team across three continents redoing the webpage.
They are crazy ideas that work, that you will into existance.