Utaite: (歌い手) is a Japanese term for people who cover previously released songs and post them on Nico Nico Douga and YouTube under the utattemita category. The term "utaite" is unique to Nico Nico Douga singers, making it different from Kashu (歌手), which means "singer" in general.
Generally, utaite do cover songs for VOCALOID original songs, anime, game-related songs, and J-Pop. They also do parodies with different lyrics for popular songs, intended for amusement.
Most utaite exclusively cover songs and upload under the utattemita section, but some of them have doujin circles and release original music/drama CDs during events like Comic Market and VOCALOID M@ster, frequently referred to as Comiket and VoMas respectively. A few utaite also have become partly or wholly professional singers and have released/will release their albums under official labels. Some examples are Valshe, Piko, clear, 96Neko, Nano, and Lon.
Occasionally, Utaite may collaborate with each other, forming pairs or groups that they collaborate in. Many Utaite have people they collaborate with often, such as clear with nero or Usa with Yamai. There are many famous duets, such as magnet and famous group songs, notably Smiling. Collaborations differ from Awasetemita (合わせてみた, lit. "Tried to combine"), more commonly known as Nico Nico choruses, in that collaborators give consent and actively participate, whereas choruses are created by a separate party after the utaite involved have uploaded their own covers.
Recently, utaite appearing in concerts and live events have also become popular and have become more prolific and public. Most of these singers perform on Nico Nico Live concerts or Nico Farre, though several like Piko, having gone major, perform in their own right.
Mixing and Encoding
Producers as Utaite
VOCALOID producers are people involved in composing original songs or tuning VOCALOID to cover existing songs. Some of these people also sing and upload their covers under the utattemita category. A famous producer who also sings is Tourai. Utattemita sung by producers in Nico Nico Douga is categorized under the tag "Utau P Series" (歌うPシリーズ) which literally means "singing producers series".
For more information on the producer, utaite see here.