An application program is a software component that makes use of services and resources managed by the underlying OS. An application program, unlike an OS, also provides specific values to the end user. For example, a word processor (Microsoft Word, OpenOffice Write and etc.) makes it easier for an end user to create, format and manage documents. A web browser (Microsoft Internet Explorer, Mozilla Firefox, Opera, etc.) enables an end user utilize web resources.
In the end, it is the applications that make a computer useful to an end user. An end user directly interact with application programs with little or no exposure to the other components in a computer.