If you want to create a repeating table within your form this requires code or qRules (as previously stated). The log is then maintained within each form and you have to open each form to see what has occured.
Alternatively you could create a separate list that has a 'log' item added to it whenever a change occurs to the form. This log item can be created from within Infopath (using web services - no code required just rules), alternatively you could create a SharePoint workflow that triggers an 'On Change' workflow (which can be somewhat easier to implement and you could also implement a reusable or site workflow - which can provide a standardised log mechanism for your lists/content types/the whole site).
Hope this helps.
Cheers