Hi! The following overview is information from a longer guide to textmode I've written here. I hope it's useful!
" Textmode is a broad term for art made with a monospaced font on a uniform grid. The term arose from a need to categorise art that didn't use a specific specification.
"In the same way that 'tile art' encapsulates pixelart, mosaic, sprite art (and textmode), 'textmode' covers a great many different subcategories. Textmode encapsulates art like ASCII, ANSI, PETSCII and many others which have their own specific restrictions.
"Old computers used to have various modes for displaying graphics - with hires modes for displaying pixelart with varying degrees of colour, and often a text mode for displaying uniform tiles of text characters in units of one character, one foreground colour and one background colour. This was beneficial because it required less RAM to display large, colourful graphics.
"Textmode is a general term for tile art which is:
Created from a fixed two colour tileset (usually a 1bit font)
On a uniform grid
With at most a single foreground colour per character
With at most a single background colour per character
Popular textmode editors:
REXPaint by Josh Ge
PLAYSCII by JP LeBreton
lvllvl by James_
NewPixelsApp (vectorised textmode aka "Vectmode" for mobile devices)
Marq's PETSCII editor
Petmate by Janne Hellsten
Moebius, a textmode editor specifically for ANSI
Notepad is probably still the best for ASCII.