Quick Overview (courtesy of Wikipedia)
An acrostic is a poem or other composition in which the first letter (or syllable, or word) of each line (or paragraph, or other recurring feature in the text) spells out a word, message or the alphabet.
Acrostics are common in medieval literature, where they usually serve to highlight the name of the poet or his patron, or to make a prayer to a saint.
They are most frequent in verse works but can also appear in prose. The Middle High German poet Rudolf von Ems for example opens all his great works with an acrostic of his name, and his world chronicle marks the beginning of each age with an acrostic of the key figure (Moses, David, etc.). In chronicles, acrostics are common in German and English but rare in other languages.
Acrostics can be more complex than just by making words from initials. A double acrostic, for example, may have words at the beginning and end of its lines, as this example, on the name of Stroud, by Paul Hansford:
S et among hills in the midst of five valley S, T his peaceful little market town we inhabi T R efuses (vociferously!) to be a conforme R. O nce home of the cloth it gave its name t O, U phill and down again its streets lead yo U. D espite its faults it leaves us all charme D.