Web applications are only meant to be used through a web browser. These are a mix of server-side and client-side scripts (ASP.NET, PHP, etc.) and (HTML, JavaScript, etc.). For the hard work in supplying the system's core functional web services, the web browser retrieves the web server components deployed at the system's backend infrastructure. Facebook, Medium, Twitter, Smashing Magazine, Flipkart, and other popular online apps are a few examples.
On the other hand, SaaS (software as a service) is a more advanced online application. It's also used to access internet services like web applications, although it's not necessarily reliant on the web browser to function. SaaS products provide a deeper user customizing experience and seamless connection with reliable and consistent backend infrastructure. Evernote, Salesforce, Hubspot, Zoho, Dropbox, and other popular examples are just a few.