According to Wikipedia, a "tag", in barbershop music, is the last section of the song. Its rough analog in Classical music is a coda.

Tags are characterized by a heightening of the dramatic tension of the song, frequently including a "hanger", or posted (held) note. In addition, good tags can be sung as short, stand-alone works. Tags may be soft and tender but are typically characterized by loud, "paint-peeling," ringing chords. According to the competition rules of the Barbershop Harmony Society, every song entered for a competition must have a tag.

When Barbershoppers get together, even if they have never met before, it is common for them to get into groups of four or more and sing a few tags. Those that don't know the tag being sung can be easily taught by those that do. This typically brings feelings of joy and camaraderie to all involved.

Barbershop is a style of a cappella (unaccompanied) singing, characterised by four male vocal parts sung in close, rich, consonant harmony. The most well-known expression of this musical style is the "Barbershop quartet", the contemporary form of which has shed its twee image of four men with moustaches, striped suits and boater hats, and is now sleek, professional, musically sophisticated entertainment. Groups of men larger than four can also sing Barbershop music. Organised groups are usually known as "choruses" and are found in most North American cities, as well as in Australia/New Zealand, Great Britain and several countries in Europe.

The organisation responsible for overseeing Barbershop singing and development in North America is the Barbershop Harmony Society (BHS). It organises an annual convention and competition for quartets and choruses, which has become the de facto world championships for the art form. Groups from affiliate organisations in other countries are also invited to compete. Contestants must adhere to strict rules governing their performance style and the musical structure and subject material of their chosen songs.

The female counterparts to the Barbershop Harmony Society include Sweet Adelines International (SAI), Harmony, Inc (HI), and others. Mixed (male and female) Barbershop groups exist, but are not common.

For more information, visit the Wikipedia page.

SATB stands for "Soprano, Alto, Tenor, Bass (or Baritone)" - the names of the voice parts of a typical four-part mixed-gender (female/male) a cappella quartet or choir. SATB groups sing a wide variety of a cappella music, not just Barbershop

A Cappella is a musical style comprising of one or more voices without instrumental accompaniment. The term is Italian (or Latin) and literally translates as "From the chapel" (although most a cappella music is not church-related).

Barbershop is one type of a cappella singing.

Multitrack refers to a recording technique. When it applies to tag-singing, it means that each of the voice parts is recorded separately and then mixed together after recording is complete (as opposed to recording several singers harmonising with each other in one recording). In a multitrack recording, each voice part can be sung by a separate individual, or one person can sing all voice parts.

The term can equally apply to video recording. During the mixing stage of a multitrack video, the video screen is typically split into sections (4, 6, 8, etc), each section showing one person singing one voice part (they can often be all the same person, resulting in an eerie effect). A good example is here.

Two articles on how to record a multitrack tag can be found on our Multitrack page.