SharePoint is called a web platform, as opposed to just a website, because of the sheer amount of functionality and capabilities it includes. In fact, if you already administer a SharePoint website, you can easily create a new website right within the existing website.
You can also develop websites with an extraordinary amount of functionality without writing a single line of code. The result is a platform for websites instead of just a single website. The multitude of features and the complexity of the product are what lead to confusion.
The terms SharePoint website and SharePoint site can be used interchangeably. Both terms mean a website that is powered by SharePoint.
One thing that makes SharePoint so special is that you don’t need to be a computer genius or even a power user to be a website developer and administrator in SharePoint. You just need to be comfortable using a computer.
The term website and web application are often used interchangeably. In the deep, dark technical world of SharePoint administration, the term web application has a very specific meaning. A web application is a technical construct, and each web application has its own databases associated with it. If you create two SharePoint web applications, they store their content and configuration information in different databases.
As with technology these days, a simple word can have different meanings, depending on the context of the conversation.