Figuratively speaking, applications sit on top of systems software because they are unable to run without the operating system and system utilities. Systems software consists of low-level programs that interact with the computer at a very basic level. This includes operating systems, compilers, and utilities for managing computer resource
Application Delivery Mechanisms
Developers have many different options for getting their applications to end users. In decades past, nearly all applications were installed directly on the users' PCs and/or servers.
Today, many applications are delivered as Web applications. The code for these applications resides on a Web server, and users access the application via a Web browser. Common examples of Web applications include Web-based email, social media platforms, wikis and online auctions.
The distinctions between Web applications and websites can be a bit fuzzy. However, in general, websites have primarily static content with few interactive elements, while Web applications have primarily dynamic content and are designed for user interaction.
A third type, software-as-a-service (SaaS) applications, is closely related to Web applications. As with Web applications, users generally access SaaS applications via a Web browser. However, some SaaS applications can also be accessed via a native mobile app on a device like a smartphone or a tablet. Also, user data for a SaaS application is stored in a cloud computing environment (which may or may not be the case for Web applications), and many SaaS applications charge a subscription fee, which is less common for Web applications.
Well-known examples of SaaS applications include Salesforce.com, Microsoft Office 365 and Adobe Creative Cloud.]
App Versus Application
Many people use the words app and application interchangeably, but purists will tell you that the two are slightly different. App is used to describe a type of application that has a single functionality, whereas an application may handle a number of functions. See Webopedia's app definition to learn more.