A Startup is easiest defined as a young company founded by one or more people that are mostly developed to bring a unique product or service to the market. As this is another umbrella term and spreads across multiple industries, there are multiple roles within a startup that needs to be filled from product development to business development to sales. Especially now, there are more and more startups being created.
The risk associated with joining a startup are that when joining a startup there is no guarantee that the startup will take off meaning there is less job security than if we work for an established brand. But as always, the more risk, the more reward. If you are working at a startup that takes off, you have a much higher career progression and you could even end up with equity in a company quickly.
The main difference between working for a startup vs an established company is that the roles are a lot less defined and you could find yourself taking on responsibilities that were not first expected of you. When joining a startup, it will be expected of you to make an impact from the day you join.