What is the concept of a virtual path provider?

Any project has two parts: the standard and common part and the customized version. In ASP.NET, we have two types of pages for any project: one is the common ASPX pages and the other is customized ASPX pages. The common pages are stored on file directories while the customized versions of pages are stored in the content database.
In other words, we need an abstract mechanism by which we can render pages from the SQL Server content database and also from the virtual directories. This is achieved by using the virtual provider provided by SharePoint. So for all customized pages, the virtual provider reads from the content database and passes them to the ASP.NET runtime. For all common pages, it goes to the directory, parses it, and the passes it across to the ASP.NET runtime.

The virtual provider is an abstraction which loads the page from the content or the file system depending on whether it’s customized or common pages, and passes it to the ASP.NET runtime.