Subtitle editor

A subtitle editor is a type of software used to create and edit subtitles to be superimposed over, and synchronized with, video. Such editors usually provide video preview, easy entering/editing of text, start, and end times, and control over text formatting and positioning. Subtitle editors are available as standalone applications, as components of many video editing software suites, and as web applications.
Purpose.
In television, subtitles are used for "clarification, translation, services for the deaf, as well as identifying places or people in the news." In movies, subtitles are mainly used for translations from foreign languages. 
Subtitles are frequently used to provide informative details regarding the action on screen, such as the names and titles of interview subjects, a discussion topic change, to spell out web URLs or email addresses, to assist understanding speakers who mumble, details in instructional videos such as recipe ingredients, and humorous effects such as commentary or contradictory captions.
Each subtitle, during editing, consists of text, a start time, an end time or duration, and optional text styling and positioning.
Time is measured either in video frames, in milliseconds, or in hours:minutes:seconds.frames/milliseconds. The exact format and separator characters(colon, period, comma, etc.) are determined by the subtitle format chosen. A capable subtitle editor can convert one time measurement system into another without error.
Editor features.
A subtitle editing tool will typically have these features:
Editors used by broadcasters may have additional features:
See the general description of subtitles for further description.
Not all editors are equally flexible in terms of video formats, subtitle formats, or ease of use.