Fail Over is a function that automatically switches to a spare system when a server, system, or network problem occurs. At this time, when a person switches manually, it is called "switch over." 

Front-end is the design of the part shown to the user on the web or app. It operates in response to the user's request by providing various contents such as documents and videos to the user. 

Framework is an open source making the development process faster and more convenient as a frame containing general functions used in many software. Developers can develop more efficiently by implementing individual functions based on the framework, but the disadvantage is that there is a limit to the range that can be customized because it has a framework. Various frameworks are provided for each programming language. 

Focal length is the distance between the center point of the lens and the focus(subject). As the focal length increases, the angle of view becomes narrower and the depth of the field becomes shallower.