Harish's SOA Blog: (WID/WPS) Human Task Overview

Task templates : Definition of a deployed task model, contains properties like name priority, escalation template, custom properties, people query templates etc.

Task instances : Runtime occurrence of a task template.

Subtask : Supports people when they need to delegate parts of their assigned work to other people, but want to keep control over the overall result.When the first subtask is started, the parent task enters the waiting-for-subtask substate. It remains in this substate until the last subtask reaches one of the end states finished, failed, expired, or terminated.
Read more : http://publib.boulder.ibm.com/infocenter/dmndhelp/v6r2mx/index.jsp?topic=/com.ibm.websphere.bpc.620.doc/doc/bpc/csubtasks.html

Follow-on tasks : Supports people when they want to delegate parts of their assigned work to other people, and the control over the completion of the work.When the follow-on task is started, the predecessor task enters the forwarded state.
Read more : http://publib.boulder.ibm.com/infocenter/dmndhelp/v6r2mx/index.jsp?topic=/com.ibm.websphere.bpc.620.doc/doc/bpc/cfollowon.html

Adhoc Task : Creating task dynamically at runtime.

The idea is to specify and use task templates dynamically, without a tooling application like WID, and without an explicit deployment step.
A task which is not part of the modeled process flow, but is created "on the fly" in the runtime environment to address work to be done.
This task can be based on a predefined task template, or a free-form task that is created using a form editor.