Choose the report you want to work with, and then hit Customize to get the Customize Report Window.  From the Preview Menu with your sample data, you have three Report Formats to choose from.

-  Tabular

-  Summary

-  Matrix

Tabular is all the details.  This is the most comprehensive view of information.  WHen you run the report, you get a row of information for every item in the report.   If the report is volunteer opportunities, occurrences and connections, for example, you'll get a row for EACH connection.   Which means the opportunities may show up many times in each row.

What if you would like to summarize information by a certain field,  Let's say, we want to summarize these field by if they were managed by Affiliate or Managed by the Partner.  I can do that with this format.  We will show you HOW to do this in the Using Summaries Training.

You can continue to Summarize information by adding Summaries IN summaries.  In this case, I have a report that summaries the Opportunity City, then the Opportunity names, and has the Occurrence Detail below it.

If I am uninterested in the individual records, I can hide the details to get some basic information on the Summary of these field by using the Show Button and un-checking the Details.  We will talk about Drop Zones Later

You can use Matrix Format to see a Grid of Two or more Summaries.    In this case, I am looking at the summaries of Volunteer Opportunity Names along the Y axis and the Populations served along the top.  Notice there are row Grand Totals, and at the bottom there is a Column Grand Total.

You can continue to add more summary rows and columns using the Customize Report Window

Salesforce.com has wonderful training and support material that can help you find specific HOW TOs on some of the more advanced Summary and Filter functions.  

Check it out here: Reports in Salesforce: Summary Formats