- Set Series - the kind with a set number of books planned from the beginning.
These are the series where every single book has a complete plot arc (or should), and then the overarching series also has a plot arc. The best example that every one will instantly understand would be the Harry Potter books. In each book there's a plot (Harry getting the Sorcerer's Stone, Harry saving Ginny from the Chamber, Harry competing in the Tri-Wizard Tournament, etc.), but the series itself has an overall plot (Harry vs. Voldemort). From the beginning, there were going to be 7 Harry books, and by golly, (even if some of them got kind of long) there were 7 Harry books. Other examples of series like this would be the Percy Jackson books, the Series of Unfortunate Events books, and locally, The Forgotten Worlds Series from CBAY.
- Open-ended Series - the kind where each book is its own stand-alone adventure.
In these books, you can have as many adventures as you can think up. The only thing that carries over are the characters. Beloved by book packagers and the ghost-writing teams, these series can literally go on indefinitely. The most famous are the various series produced by the Stratemeyer Syndicate (Nancy Drew, Hardy Boys, Bobsbey Twins, etc.) However, there are also modern day versions like The Babysitter's Club (who may or may not be written personally by Martin -- I won't pretend to know) and many chapter book series written by a single author like the Magic Treehouse Books and, my favorites, the Judy Moody books.
- One paragraph summaries
This is where you write a one paragraph summary for each one of your proposed books in the series. In these small summaries you need to detail the major plot arc in its entirety. So, yes, you will need to give the plot away. This type of summary is appropriate for both set and open-ended types of series. However, if you are doing an open-ended series and dream of some day producing 46 books, do not try to think up a summary for 46 books. Limit yourself to around 5 for now.
- Full page summary
This is where you write a one page summary of your the overreaching plot of your series. Exactly like a one page summary of a single book, this is a short-story summarized version of what the overall series plot arc will be. Obviously, this style of summary only works with a set series.
So, over at the forum I have set up a board for folks to practice their series summaries. And even if you aren't working on a series at the moment, this might be a good time to make one up. You never know when you might have characters you are so in love with that you want to keep writing about them.