A stab at some terminology

Step

  Steps are artifacts in a pipeline document. The <p:pipeline>,
  <p:step>, <p:(for-each|viewport|choose|...)>, and <p:declare-input>
  elements are all steps. Steps have a type and identity (and
  input/output/parameter bindings).

  Steps either declare a single component or or instantiate one or more
  components.

Step Type

  The type of a step is essentially it's localName in the XML syntax.
  So we have "step" steps (ordinary steps, if you will), "for-each" steps,
  etc.

Step Identity

  The identity of a step is its name which must be unique within a
  pipeline.

Step Containers

  Some steps are step containers.  A step container is just like any other
  step except that it contains steps of its own. Special rules apply to
  the input/output (and maybe parameter) bindings of steps in a step
  containre.

Component

  Components are artifacts in the "flow graph" that describes the operation
  of a pipeline. Components have a component type and identity (and
  input/output/parameter bindings)

Component Type

  The type of a component is essentially the operation it performs,
  e.g., "an XSLT component" or "a validation component". Step
  containers (I think it's only step containers) instantiate
  components with a unique, anonymous type that matches the declarations
  on the container.

Component Identity

  All instantiated components are unique. (e.g., an implementor might
  write code to implement a component of type XSLT and a running
  pipeline might contain two instances of that componet.)

Pipeline Library

  A pipeline library is a collection of pipelines.

Any good?

                                        Be seeing you,
                                          norm

-- 
Norman Walsh
XML Standards Architect
Sun Microsystems, Inc.