Conducting a choir

Conducting refers to directing a musical performance. Conducting a choir takes more than just standing in front and waving your hands to a particular rhythm, you have as a conductor the primary responsibility of setting the tempo, ensure correct entries by various members of the ensemble, and to "shape" the phrasing where appropriate. The conductor is in charge of the choir while on stage and must make sure all entries and exists are well timed. 
It is possible that a conductor can deviate from original arrangement previously rehearsed. Well, when this happens, (it is expected that before the performance, the choir already trust the conductor to lead them safely to the end of the piece), the choir should still follow the conductor. This is to ensure uniformity.
A conductor communicates with their musicians primarily through hand gestures, typically though not invariably with the aid of a baton and may use other gestures or signals, such as eye contact with relevant performers. the hand gestures are mostly dependent on the time signature of the  piece.  
It is true that the conductor should also include the dynamics and gesticulations to his art, but its should not and must not be at the expense of his core responsibility.

A baton
A conductor's directions will almost invariably be supplemented or reinforced by verbal instructions or suggestions to their musicians in rehearsal prior to a performance.
The conductor typically stands on a raised podium with a large music stand for the fullscore, which contains the musical notation for all the instruments and/or voices. 
Depending on the size of the choir, a conductor's position may also vary. Normally a choir less than 10 may not need a conductor. In this case Soloist, or Instrumentalist act as guide for the choir.
Conductors act as guides to the orchestras and/or choirs they conduct. They choose the works to be performed and study their scores, to which they may make certain adjustments (e.g., regarding tempo, articulation, phrasing, repetitions of sections, and so on), work out their interpretation, and relay their vision to the performers.