From the longer Wikipedia page [1]
A calligram is a text visually arranged in a way that it forms an image associated with the text's contents. It can be a poem, a phrase, or a single word; the visual arrangement can rely on certain use of the typeface, calligraphy or handwriting, for instance along non-parallel and curved text lines, or in shaped paragraphs. The image created by the words illustrates the text by expressing visually what it says, or something closely associated; it can also, on purpose, show something contradictory with the text or otherwise misleading.
Guillaume Apollinaire was a famous calligram writer and author of a book of poems called Calligrammes.
There are various websites and pages on the subject.