They're independent from each other. A startup is young company/business offering a product or a service. A web app can be the service that a startup provides,

They're independent from each other. A startup is young company/business offering a product or a service. A web app can be the service that a startup provides, which is why you see many web startups as web apps. I can create a web app without making it a startup/business, and vice-versa.