First off, look in the CRM SDK Help for Entity Relationship Behavior. That provides the fundamentals of dealing with parent/child relationships.
Depending on where you start will also drive what behavior you use. If you start with the parent entity and want to update child entities, you need to RetrieveMultipleRequest through the entity relationship to pull all related child entities. Example
here:
http://msdn.microsoft.com/en-us/library/microsoft.xrm.sdk.messages.retrievemultiplerequest.aspx
If you're going the other direction, use RetrieveRequest across the relationship ID:
http://msdn.microsoft.com/en-us/library/microsoft.xrm.sdk.messages.retrieverequest.aspx
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