"Startup" means many things to many people, but most people most of the time use it to mean something close to a rapid-growth business (millions in revenue in < 3yr from start, growth measured in integer multiples per year from there), commonly targeting revenues such that (anecdotally) maxing out in the low $millions means the startup will likely fail for financial reasons. Creating a business that can do this frequently means taking outside investment from speculative investors (VC, angel) in numbers ranging from the low six figures for young, unproven startups, to tens of millions of dollars for established startups with obvious room to grow. You spend that money doubling your headcount every year and possibly doing things like subsidizing freemium users.

Crucially, the startup might not be like a business in some important regards. It's common to operate a startup at a significant financial loss, with the goal of either being bought out or growing fast enough to "make it up in volume" before the VC money runs out.

(sidenote: the word "startup" is also coming to sometimes be used to mean a self-funded tech-oriented business, without VC backing, with a slower expected growth rate but an expectation of maxing out at significant revenues).

All of this is in contrast to "small businesses", which have relatively modest growth prospects (sub-$1m is very common), frequently keep a headcount low enough to gather all employees in a small room, and are designed with prompt profitability in mind from the start, due to taking little/no outside investment.

Startups, both by their mode of operation and their level of ambition, rapidly grow to hundreds or thousands of employees, and demand immense revenue/funding to cover that. Small businesses are designed to get off the ground efficiently and more or less coast, sometimes with limited growth over time, sometimes keeping a steadystate.