A start-up is a company that is literally just starting up. That is to say, it is brand new. The difference between start-ups and other longer existing companies is that a start-up is still in a phase of development and research for markets. This means that there is not yet a clear path in which the company operates, there are no ready-made solutions for problems and success is not yet guaranteed. As a quant, you can be the instrumental part of a start-up, implementing a new idea on a grand scale and streamlining this implementation in the market.
Quants are notoriously important in any start-up, providing a much needed quantitative perspective on exciting new ideas. Working at a start-up will be extremely rewarding when you are looking for a environment where your opinion matters in the fundamental decision making of a company, but also involves a less clear-cut job description. For those who enjoy finding new and creative ways of developing a company, start-ups are the way to go.