respond_to

Defines mime types that are rendered by default when invoking respond_with.

respond_to :html, :xml, :json

Specifies that all actions in the controller respond to requests for :html, :xml and :json.

To specify on per-action basis, use :only and :except with an array of actions or a single action:

respond_to :html
respond_to :xml, :json, except: [ :edit ]

This specifies that all actions respond to :html and all actions except :edit respond to :xml and :json.

respond_to :json, only: :create

This specifies that the :create action and no other responds to :json.