A taskmaster is an NPC who assigsns tasks to PCs as the means of advancing in a guild. These NPCs are generally guildmasters and trainers, but this isn't necessarily true. They will only assign tasks for one guild.
Ask taskmaster about task is the way to get assigned a task. Upon completion, the same syntax is used.
Some tasks require you to obtain an item for the taskmaster. If this is the case, you can give taskmaster when that item is held in the right hand to compete the task.