Nyan or nya is the Japanese word for meow (the cat cry): thus, the title means Meow-meow-meow-meow-meow-meow!. The song is an endless loop of Miku saying "Nya" over and over while pouncing. Back in 2010, this song was obscure to those outside the VOCALOID and UTAU fandom, until Momo's version came into light.
On April 5, Saraj00n uploaded video of a Poptart Cat with UTAU Momone Momo's voice on YouTube. The song became popular, under the name of Nyan Cat. It quickly spread around adn became an internet meme appearing in multiple sites, blogs, and social networks. It quickly rose to fame over the next few months, and there it started about the origin of the song. The meme has become so popular that daniwell has accepted the name Nyan Cat as the name of an official album.
Non-VOCALOID fans (fans of the Nyan Cat video) and VOCALOID fans debated on the originality, some incorrectly claiming Nyan Cat came first, while the latter correctly claimed the song was by daniwellP. It caused flame wars for Japanese fans on the supposed Nyan Cat video on niconico, telling them to "get lost" or to speak Japanese, as it is customary in NND to speak Japanese.
It was during that time a few people visited daniwell's original video and Momo's niconico Douga video, helping it getting a lot views. At the same time, it helped spread the VOCALOID fandom around the West even more.
The confusion died down after a few months prior to the Nyan Cat Video, but it left a bad impression on both sides.
A simple loop animation done by Eromame. Basically is the same simple dance of the original PV, just drawn in his style and with Tako Luka flying occasionally in the background. It was made as a Special PV for the Exit tunes Album, Vocalonation
MomomomoP (ももももP) used Momone's voice to create this version. It was this version that was used to co-create Nyan Cat. The niconico video was a sight for multiple users; users from around the world commented in several languages on the video, leaving Japanese users frustrated with the sudden multilingual comments scrolling on screen.
Fanmade PV (CGI, MMD)
A 3DPV take on the song. It shows Miku with her headset shaped like cat ears, and does a cute dance.