Appointment scheduling software allows businesses and professionals to manage scheduling appointments and bookings.

This type of software is also known as appointment booking software and online booking software.

A control element (sometimes called a control or widget) in a graphical user interface is an element of interaction, such as a button or a scroll bar.

Controls are software components that a computer user interacts with through direct manipulation to read or edit information about an application.

User interface libraries such as Windows Presentation Foundation, GTK+, and Cocoa, contain a collection of controls and the logic to render these.

An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs.

All computer programs, excluding firmware, require an operating system to function.

Freeware is software that is available for use at no monetary cost.

In other words, while freeware may be used without payment it is most often proprietary software, as usually modification, re-distribution or reverse-engineering without the author's permission is prohibited.

Two historic examples of freeware include Skype and Adobe Acrobat Reader.

There is no agreed set of rights or a license or an EULA which would define "freeware" unambiguously; every Freeware publisher defines their own rules for their Freeware.

For instance, redistribution of Freeware by third-parties is often permitted but there is a significant portion of Freeware which prohibits redistribution.

A client is a piece of computer hardware or software that accesses a service made available by a server.

The server is often (but not always) on another computer system, in which case the client accesses the service by way of a network.

The term applies to the role that programs or devices play in the client–server model.