TFS Work Item Links and Relationships
Parent/child —This is a useful link type for representing hierarchies such as feature catalogs, or for detailing task work items (children) that 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.
Tests/tested by—This link type is primarily intended to model the relationships between test-case work items and the requirements or user stories that they test. This makes it easier to determine the quality of a given requirement or user story by examining the recent results for its related test cases. A work item can test zero or more work items.
Successor/predecessor —The successor/predecessor link type is used to indicate work items that have a dependency relationship with one another. For example, designing the user interface for a web page is generally a predecessor to writing the code and markup that will provide the implementation of that web page. A work item can have zero or more successor and/or predecessor links to other work items.
Related —The related link type is the same as the legacy linking system found in Team Foundation Server 2005 and 2008. These link types are not directional and provide no additional context about the type of relationship. If you had linked work items in a project that was upgraded to Team Foundation Server 2012, those relationships will be represented by the related link type.
A work item is the basic building block of the project management capabilities in Team Foundation Server. Microsoft defines a work item as . . . a database record that Team Foundation uses to track the assignment and progress of work.