A billion-dollar fitness start-up that came out of nowhere could kill SoulCycle

Most clients attend its spin classes in the comfort of their own homes. They've never even met an instructor in the flesh.

Peloton is a high-tech indoor cycling company that was recently valued at around $1.25 billion after closing a $325 million financing round. Its core products are an indoor bike and a subscription-based video streaming service.

And as the business expands, it's becoming a real threat to rivals like SoulCycle and Flywheel.

Here's everything you need to know about the billion-dollar fitness company.