During the medieval period, there were several different types of heralds.
1) The term 'herald' could refer to a person making important (often royal) public proclamations or declarations of the news in the streets or squares of a town or city. This sort of person was also likely to be referred to as a town crier.
2) A court official who delivers messages of state (such as treaties) between sovereigns. Essentially a messenger or diplomat.
3) The official that formerly declared challenges between combatants and made proclamations at tournaments.