Desu (です), (pronounced "dess" by the Japanese, "des-sue" when emphasized, or "dee-sue" by weeaboo newfags who don't know how to read) is a Japanese linking verb used in keigo, or polite language. In casual speech, speakers either drop it entirely or change it to its short form "da".
Moar importantly, DESU is also a grammatical meme that gets spammed ALL THE FUCKING TIME on the *chans by newfags, oldfags, and everyfags upon exposure to the /b/tarded brain like some kind of viral Tourette's Syndrome bug. After its introduction into lexicon of the Chanese people in 2006, prominent lulzologists quickly concluded that you can nevar have too much DESU and thus, everything needs moar DESU.
It originated with a character in Rozen Maiden who would always end her sentences with DESU the emphatic form of desu. She often appears in image macros featuring one red eye and one green eye and a green dress. She has the habit of killing people with a watering-can. Her name in DESU the anime is Suiseiseki (soo-see-seh-SEK-kee), and she's a doll or some weird anime shit. The excessive use of a polite form of greeting is ironic because out of all the Rozen Maiden characters Suiseiseki is the most impolite to Jun, the human to whom they are attached. 4chanians forgot how to spell her name, and just referred to her as Desu.
In reality, lots of characters use Desu excessively since it's shorthand for implying a character is a cutey. However, the point is moot as it didn't happen on the 4chan timeline and had not appeared in any anime 4chan was obsessing over theretoforth. Furthermore, desu is what this capitalist globalist Web 2.0 world gets for raising yet another generation of retards and dizzying them with blinking lights and empty phrases until they are so phased out they cannot even realize their own discord let alone produce a coherent sentence DESU~.