A "cinquain" is a five line stanza and it comes from the words "cinq" (French meaning "five") and "quinque" (Latin meaning "more at"). The five lines make up the entire poem but to make it a cinquain, it has to adhere to a strict formula of syllables in which syllable count plays a specific and cruel role :

line 1 - 2 syllables
line 2 - 4 syllables
line 3 - 6 syllables
line 4 - 8 syllables
line 5 - 2 syllables 

Kit Thompson is so happy someone took the time to organize this issue but it goes a bit deeper for me. Basically, I just dig the word and find it charming that there are names for even the most mundane things. In Faribault, we would call this a five-line poem and head off to Basilleo's content with our brevity-loving knowledge, but I digress. However, calling it a "cinquain" is much more high-toned and worth noting for the reading cyber-public. You know, vox populi and all that.