Applications are ubiquitous on the Internet, and understanding how they work is paramount to understanding how to build them.
Building applications in SharePoint means building sections of a page that interact with each other.
SharePoint has a built-in infrastructure for building applications, and in fact ships with some very functional applications out of the box.
Blog sites have code built into them that allows automatic one-to-many relationships to exist in SharePoint.
Building a simple application can be as easy as dragging and dropping items inside your favorite Web browser.