[Zope] ZCatalog, FieldIndexes and TextIndexes

Can someone give me a good explanation of the difference between a FieldIndex and a TextIndex? Also... I have created a ZCatalog for a search on my web site. Currently, the users can search on Title, Keyword, or Summary, but they have to choose which one they want to search on. I want the search to have one entry box that will search all three different fields. I'm sure there's a good way to do this, as I see it on the zope.org site, but I can't figure out how they did it. Any ideas/help would be appreciated.