"Task Structure" = the logical arrangement of the entries in the task list.
Maybe I'm inventing terms but when I look at the task list in MS Project it
seems logical to start with the biggest work package "Build House" and break
it down into smaller and smaller work packages - Secure Site, Lay
Foundation, Erect Walls .... - until you get to the individual activities
done by individual resources or resource teams. The key is always
maintaining focus on keeping everything in the outline a description of the
observable physical activities being performed by one or more warm bodies.
A Work Breakdown structure always focuses on the work being done - it
focuses on the *actions* producing deliverables that are being performed by
resources . In contrast, a Bill of Materials type of structure focuses on
detailing the physical components of the final deliverable - it focuses on
the *things* the actions produce.
"Spagetti links" would be something similar to the spagetti code one
sometimes encouters in programming with modules calling other modules which
call others ad infinitum. Spagetti links would describe a case where tasks
with dependency links are not in reasonable physical proximity to each other
in the task list and so the link lines have to wind in and out and over and
around intermediate tasks to get from a task to its sucessor laying pages
away in the chart. If all the activities associated with laying the
foundation are subtasks of the Lay Foundation summary they are going to be
reasonably close to each other. But if the breakdown is arranged instead
with summaries for modules such as Structure, Piping, Electrical, HVAC,
Interior, Decorating etc etc you might well find a link from something in
the Structure summary having to find its way to a successor under the HVAC
summary pages and pages away. Not that links from an embedded task in one
summary might not ever find its way to a task in another summary but the
logic behind the way the work is broken down can influence how much of it
occurs.
According to the PMBOK, a WBS is a "deliverable-oriented grouping of project
elements that organize and define the total work scope of the project.
Each descending level represents an increasingly detailed definition of the
project work." It is the document that completely defines the project
scope - everything that must happen to complete the project must be listed
and any action not listed therein lies outside the scope. (As an aside,
this is why I'm not thrilled at the notion of "adminstrative tasks" like
vacations and such - while they certainly influence resource availability,
they lie outside the project scope itself in that they don't in themselves
contribute to the final deliverable and therefore they don't belong in the
WBS.) That is contrasted with an Organization Breakdown Structure (OBS)
which shows which portions of the scope have been assigned to which
organizational units, a Resource Breakdown Structure (RBS) which shows what
portions of the scope have been assigned to which specific individuals, and
a Bill of Materials (BOM) which "presents a heirarchical view of the
physical assemblies, subassemblies, and components needed to fabricate a
manufactured product." People who use a BOM model instead of a WBS model to
organize the task list in MS Project are trying to use the manufacturing
model to assemble a one-off product that is the project deliverable.