All applications can be broken into three layers Presentation Layer - What the user sees.
Most applications have requirements that span all three of the layers.
However, as a developer, it is usually better to make sure that within the design of your application, that you separate the three layers. Well, mainly because things change. Especially in the world of computers. In fact, things don't just change, they change extremely quickly and often dramatically. As a result, developers must often modify applications to meet new requirements in short order.