Salesforce provides a number of reports that combine related objects.  These 'standard' report types are fine if all the info you want to get are contained in the objects named in the reports.  You cannot however, modify these report types or customize them by adding lookups to other fields, etc.  For maximum flexibility in your reporting, you should work with custom report types:

These report types have been added as unmanaged so that you can add custom fields, change field labels, and make other edits as needed. System Administrators can edit and modify these unmanaged report types as needed. See post on managing Report types.

HOC Connections and Contacts- This allows you to build reports off of connections that includes information from the related Volunteer Opportunity, Occurrence, and Contact.

HOC Contacts with/without connections- This allows you to build reports off of contacts with or without connections

HOC Organizations with Primary Contact Information- Allows you to report on Organizations and their Primary Contacts.

HOC Organizations, Contacts, and Connections- Used to get information based on contacts with connection records.

HOC Vol Opps and Occurrences with/without connections- Used to build reports for all Volunteer Opportunities and Occurrences in the system regardless of volunteer connections.  You can always add a cross filter to a report of this type if you're only interested in those volunteer opportunities that are WITH connections.

HOC Volunteer Teams, Members, and Contacts- Allows you to report on information from the Volunteer Team, Team Members and pull in the volunteer's contact information.

We recommend working with these report types (rather than the standard ones that Salesforce creates from the objects), as they provide more flexibility for customizing your reports going forward.