An ASCII owl is a picture of an owl composed of ASCII text characters. The variety of characters allows the artist to create a detailed rendering.
ASCII is a text standard that makes letters, numbers and special characters universally machine readable and printable. ASCII artists choose the characters and then key them into a document in a way that creates the owl's shape and such details as wings and a beak.
ASCII art ranges from rudimentary line drawings to highly intricate pieces. Numerous websites have instructions for individuals who'd like to produce their own images. Alternatively, completed ASCII art can be downloaded for free and inserted into documents.