Autoresponder's answer calligram is absolutely correct but for the sake of completeness, the other forms of art whereby images are created from repeated keywords or significant phrases using different typeface sizes and fonts are called:
From Art.com
Similarly, expression or
representation of objects and people trumps the legibility of written
words, as in Sean Williams’ artwork, weaving words and negative space
into typographic portraits or The Experiential design collective’s
portrait of Barack Obama for TIME Magazine, which was made entirely of
headlines from his presidency.
Text portrait of Winston Churchill by Juan Osbourne