A text editor is a type of computer program that edits plain text. An example of such programs is "notepad" software (e.g. Windows Notepad). Text editors are provided with operating systems and software development packages, and can be used to change files such as configuration files, documentation files and programming language source code.

There are important differences between plain text (created and edited by text editors) and rich text (such as that created by word processors or desktop publishing software).

Plain text exclusively consists of character representation. Each character is represented by a fixed-length sequence of one, two, or four bytes, or as a variable-length sequence of one to four bytes, in accordance to specific character encoding conventions, such as ASCII, ISO/IEC 2022, Shift JIS, UTF-8, or UTF-16. These conventions define many printable characters, but also non-printing characters that control the flow of the text, such as space, line break, and page break. Plain text contains no other information about the text itself, not even the character encoding convention employed. Plain text is stored in text files, although text files do not exclusively store plain text. Since the early days of computers, plain text was (once by necessity and now by convention) generally displayed using a monospace font, such that horizontal alignment and columnar formatting were sometimes done using whitespace characters.

Rich text, on the other hand, may contain metadata, character formatting data (e.g. typeface, size, weight and style), paragraph formatting data (e.g. indentation, alignment, letter and word distribution, and space between lines or other paragraphs), and page specification data (e.g. size, margin and reading direction). Rich text can be very complex. Rich text can be saved in binary format (e.g. DOC), text files adhering to a markup language (e.g. RTF or HTML), or in a hybrid form of both (e.g. Office Open XML).

Text editors are intended to open and save text files containing either plain text or anything that can be interpreted as plain text, including the markup for rich text or the markup for something else (e.g. SVG).

Before text editors existed, computer text was punched into cards with keypunch machines. Physical boxes of these thin cardboard cards were then inserted into a card reader. Magnetic tape, drum and disk card image files created from such card decks often had no line-separation characters at all, and assumed fixed-length 80- or 90-character records. An alternative to cards was Punched tape. It could be created by some teleprinters (such as the Teletype), which used special characters to indicate ends of records. Some early operating systems included batch text editors, either integrated with language processors or as separate utility programs; one early example was the ability to edit SQUOZE source files for SCAT in the SHARE Operating System.

The first interactive text editors were "line editors" oriented to teletypes: teleprinter-style or typewriter-style terminals that mechanically printed both input and output on the same continuous roll of paper, without an illuminated display. Commands (often a single keystroke) effected edits to a file at an insertion point, that the operator needed to keep track of, called the "cursor". Edits were verified by typing a command to print a small section of the file, and periodically by printing the entire file. In some line editors, the cursor could be moved by commands that specified the line number in the file, text strings (context) for which to search, and eventually regular expressions. Line editors were major improvements over keypunching. Some line editors could be used by keypunch; editing commands could be taken from a deck of cards and applied to a specified file. Some common line editors supported a "verify" mode in which change commands displayed the altered lines.

An example configuration, circa 1975, was a Teletype Model 33 as a console to a PDP-11 using Version 6 Unix, manipulating text with the ed, the standard UNIX text editor.