Most singing involves shaping the mouth to form words, but types of mouth music which use open sounds or nonsense syllables ("vocables") also exist, for instance scat singing or yodeling. Solfege assigns certain syllables to notes in the scale.
Singing can be heard in many different places, since anyone who can speak can sing. It can be informal and just for pleasure (for example in the shower), or very formal, such as singing done professionally in a performance or in a recording studio.
Singing with no instrumental accompaniment is called a capella. However, the Choral Journal and other vocal-related publications actively discouage the use of this term and prefer the word "unaccompanied".