Do you need to find what changes were made from one Civi version to another - say 4.6.6 to 4.6.9? Where can you find that?
Go to the 'Jira' - the issue tracking platform used by the CiviCRM project - at https://issues.civicrm.org/jira/browse/CRM/?selectedTab=com.atlassian.jira.jira-projects-plugin:changelog-panel
On Jira at issues.civicrm.org, you can get the specific version that issues have as "fix for": https://issues.civicrm.org/jira/issues/?jql=project = CRM AND fixVersion = 4.6.10
(Yes, those are spaces in the URL--your browser will replace them with "%20" for you.)