A Pōwhiri is a way of greeting visitors into a marae, and is done so when the visitors first arrive. It is also used for other places such as workplaces for welcoming a new employee or special events.
The Kaikaranga (caller) is a role given to an individual (usually female) from the marae to perform callings to the Manuhiri (visitors) during the Karanga stage of a Pōwhiri. [See "5 Key stages of a Powhiri" for img]
The Tane of a Marae is needed for the process of a Wero (challenge) which is performed to reveal the visitor's visit purpose.
This role is always assigned to the strongest and most skillful with weapons as this is a very important and (somewhat) dangerous task. [See "5 Key stages of a Powhiri" for img]
The Kaiwhakaatu is a person belonging to the Manuhiri (visitors), and also does calling during the Karanga stage. Their role is to respond to the questions of the Kaikaranga and call their visitation purpose.
The Kaikōrero's role is basically a speaker for the Manuhiri (visitors), and will do the majority of talking and speeches during the duration of the Marae visit.