Summary fields work with summary parts on the layout and sorting.
Summary Parts are by **A Field** ie a field that you are going to sort by. When you do so... a summary field on that Part will break down the data by what you have sorted by.
Summary parts can be Trailing or Leading... ie before or after the body.... or BOTH.... which gives you a nice bold heading at the top and a subtotal at the bottom of a subset of records. You can have many summary parts and sort correspondingly to create very sophistocated reports.
Summary Fields in Footers will behave differently depending if you have selected a running count or not.
Summary fields in Grand Leading or Trailing summaries will give you ALL FOUND RECORDs totals.
The following forum seemed to help me a little more, maybe it was the same thing just worded different. With a few more details. I'll post the link here for anybody interested.