I will use the Property__c and Broker__c sobjects. When a relationship is created between these two sobjects, the following fields are created in salesforce schema of these objects.
Property__c (child):
global Id Broker__c;global Broker__c Broker__r;
Broker__c (parent):
global List<Property__c> Properties__r;
Upward traversal: get fields of parent object for a given child object
Downward traversal: get fields of child object for given parent object
Reference: http://11concepts.com/sfdc/blog/understanding-__c-and-__r/
