To create a ClickLink relationship:
Concepts
Working with ClickLink Rules
Field Type Compatibility Matrix
Mapping a Lookup Field to a Lookup Field
Automatically Synchronizing Records
Tasks
Creating a Rule
Importing and Exporting Rules
Adding Custom Fields to Objects
Creating a ClickLink Rule
Creating Detail Level Rules
Creating Buttons and Modifying Page Layouts
Creating an Apex Trigger
Setting up Email Templates
Synchronizing Source and Target Records
Reference
Rule Fields
Mapping Fields
Relationship Fields
Button Fields
Field Compatibility with Synchronization