customtaskson the Alexa Skills Kit developer forum.
A task is a piece of user experience that is implemented in an Alexa skill, such as an action that a skill performs. When a skill receives a task request, it can fulfill the request by performing the task itself, or it can use a skill connection to allow Amazon or a provider skill to handle the request. You can combine tasks to create richer user interactions that use, for example, multi-turn dialogs or device screen cards.
A task consists of a task definition and a task implementation. A task definition specifies the name and other attributes of your task. A task implementation specifies the behavior of the task.
Custom tasks and Amazon predefined tasks
A task can be a custom task or an Amazon predefined task.
A custom task is one that is defined and implemented by a custom skill. You can create a custom task and incorporate it into your provider skill. Custom tasks allow for a wide range of possible functionality. When the provider skill is published after certification, its custom tasks can be made available to requester skills. For more information about custom tasks, see Implement Custom Tasks in Your Skill.
An Amazon predefined task is one that is defined by Amazon and performed by a skill selected by Alexa or one that's selected by the requester skill. For more information about Amazon predefined tasks, see Implement Amazon Predefined Tasks in Your Skill.