Revit Groups are like AutoCAD blocks. Draw a couple of lines and group the selection. Copy multiple instances of the group and then edit one of the grouped objects. You'll notice that the changes you made will propogate to all the other similar groups. Go to File... Save to Library... Save Group and Groups will be saved to RVG files. We use Detail Groups a number of ways.
Let's say you want to add multiple line text like revision dates to your titleblock. Just add the dates with the text tool to one of your sheets and group the new text. Copy the group (Ctrl+C). Go to another sheet. Now go to Edit... Paste Aligned... Current view and Revit will paste the text in exactly the same spot on your new sheet. Do the same for all your sheets. Now you can edit your grouped text from any sheet and the changes will propogate to all the other sheets.
Now let's say you want to build a library of Typical Details. Draft your typical detail in a Drafting View and group the selection. Go to File... Save to Library... Save Group and Groups will be saved to RVG files. At the moment there is no thumbnail viewer for RVG files so plan on creating hardcopies in a details binder or you can create Detail Project files with all your roof details in one project file. With a dedicated Detail Project file you can copy and paste drafting views from one project to another.
I understand Revit 9 will have a Details Library and tools for saving and reusing Drafting Views.