The word “acrostic" comes from the Greek words “akros" (outermost) and “stichos" (line of verse). Usually, the word or message spelled out by the first letters of each line is the overall subject of the acrostic poem.
Here is an example of a simple acrostic poem:
Dedicated to me Always there when I need him Deserves my love and respect!
An acrostic poem in which the initial letters spell out the alphabet is called an "abecedarius." Interestingly, there are several abecedarian poems in the Bible (based on the Hebrew alphabet). Examples can be found in Psalm 119 and Lamentations.
You can also find more modern examples of acrostics. The final chapter of Lewis Carroll's Through the Looking Glass: And What Alice Found There contains a poem entitled “A Boat Beneath a Sunny Sky," which turns out to be an acrostic of the real Alice's name: Alice Pleasance Liddell.
The Dutch national anthem ("The William") is also an acrostic, in which the first letters of its 15 stanzas spell Willem Van Nassov — one of the hereditary titles of William of Orange, who uses the poem to introduce himself to the Dutch people.
Although simple acrostic poems feature a word or message created by the first letter of each line of poetry, more adventurous poets have come up with double acrostics (words formed by the first and last letters of each line) and even triple acrostics (words formed by the first, middle and last letters of each line)!