<div class="MsoNormal" style="margin:0 0 10pt;">A work item is a single unit of work which needs to be complete. Microsoft defines a work item as database record that Team Foundation uses to track the assignment and progress of work.</div>Identification and establish relationship between tasks is highly important to make effective planning. It is also helpful for a project to identify dependent, relational task and tracking task. In TFS .it is possible to

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<div class="MsoNormal" style="margin:0 0 10pt;">Parent/child:</div>Using this you can make hierarchies of business requirements or for detailing task work items which will be used to implement a requirement or user story (parent). Any work item can have zero or more child work items and zero or one parent work item.

<div class="MsoNormal" style="margin:0 0 10pt;">Tests/tested by:      </div>Tests/tested by are two link type which make relationships between test case work items and the requirements or user cases or functionality  that they test. It will be helpful to determine the quality of a given requirement and scope of requirement test case cover.

<div class="MsoNormal" style="margin:0 0 10pt;">Successor/predecessor:</div>These link types are clearly identifying the dependencies between work items. Whenever anyone work on a task then it is visible to him that any prerequisite or post task is present or not.  This is mostly used for plan and track project tasks and their Dependencies

<div class="MsoNormal" style="margin:0 0 10pt;">Related </div>It does not indicate any directional relationship. If you had linked work items in a project that was upgraded to Team Foundation Server 2010, those relationships will be represented by it.For making Non-Hierarchical Relationships this link is used .