An acrostic is a poem or series of lines in which certain letters, usually
the first in each line, form a name, motto, or message when read in
sequence.

Example:
 
No light shining down
In the darkness I drown
Gloomy creatures that I fear
Hell seems very near
There is nothing to be found
Shadows make no sound

The first letters of the poem spell N-I-G-H-T-S