An acrostic poem is created by arranging the first letter of each line so that they flow in alphabetical order, or form a word or phrase. Rhyme and meter aren't issues in this style of poetry, which causes some to consider it not poetry at all. Also, the only set length is that of what you choose to represent with the first letters. However, it's been around for thousands of years. This style was common among the Greeks and derived from the Greek words akros, "at the end," and stichos, "line". It was used by Latin playwrights and Medieval monks; it was popular in the Middle High German and Italian renaissance periods. If it please ya, learn more from the links below.

An example:

Mounting the sky, higher and higher;
Oblivious to we who dwell below;
Only she feels the warmth of the sun
Now that he has set.

The subject of this poem is the "moon", as indicated by the bolded letters.

Links:

http://humanities.byu.edu/rhetoric/Figures/A/acrostic.htm
http://members.fortunecity.com/mrk/help/acrostic.html
http://www.emory.edu/ENGLISH/classes/Handbook/acrostic.html