I'm often asked what's the difference in series and trilogies. Here are some quick and easy ways of looking at your work so you know how to pitch it:
Four or more books with the same main protagonist(s). A series may have a multi-book character arc and continuing secondary story lines that can be related to the suspense (i.e. a secondary suspense/mystery such as a missing child or spouse or unsolved murder) or the character relationships with a partner, relative, spouse, significant other, etc. (Tess Gerritsen, Michael Crais, Karin Slaughter, Lee Child, Michael Connelly, Janet Evanovich, Linda Fairstein, J.D. Robb, etc.) While urban fantasy has brought more true series into the romance genre, there are few actually SERIES in romance. Romance readers generally prefer a hero/heroine who have a romance that is satisfying by the end of that novel. JD Robb has done an amazing job of growing Eve/Roarke's relationship over multiple books so it hasn't become old and tiresome (at least not for me!) She's using their very intriguing and tortured characters to give them real conflict that usually relates to the mystery they face in the book.
My Seven Deadly Sins is a series because it takes the same core group of characters and they are the protagonists through all seven books.
I'm launching my Lucy Kincaid series. She has a love interest (Sean Rogan) but there's not an HEA at the end of the first book. Instead, the relationship will grow over the course of multiple books. If I ended with two books, it would be a stand-alone and sequel; if I ended with three books it would be a trilogy. Series are particularly popular in mysteries. Some suspense/thriller writers also write series, and urban fantasy has as I said brought the series more boldly into romance (though it has been done before, it's rare.)
Many romance writers of all genres write connecting stories. These are books that are written in the same "world" and have recurring characters, but a different hero/heroine in each separate story. While your heroes and heroines and other characters may come back in other books, they will never again take the lead role. They were always be secondary characters before and after "their" book. This is the type of "series" that romance writers in particular have developed because romance readers don't read a traditional "series" (above.) While each of my trilogies have their own theme and recurring characters, all 12 of my romantic thrillers are loosely connected. Readers can read them separately and not be lost (as they might be in a true series.) A connecting series can be 3 or more books.
Authors who write loosely connected series: myself, Karen Rose, Roxanne St. Claire (the Bullet Catchers--the head of the agency, Lucy Sharpe, is a staple and I think she's in every book, but there's a different Bullet Catcher as the hero in each book), many historical authors use the same "world" and backdrop characters (I don't read a lot of historicals.)
A trilogy MAY have the same primary character(s) as the protagonist through all three books, or they may have the same characters but with a separate h/h in each book. Nora Roberts is the master of the tightly connected trilogy. The Three Sisters Island trilogy, for example, developed the three heroines as friends, and all three were in each book, but each book highlighted one of them. Nora's trilogies "stand alone" in that they do not connect (usually) to other trilogies. Often, however, her trilogies are so interconnected that you really need to read that particular trilogy in order to fully enjoy the stories and character development.
You can do almost anything in the trilogy, which is why it's a favorite of readers, writers and publishers. If it fails, the publisher and author haven't invested in a series that might have loose ends. If it succeeds, you can build on it or build on something completely different.
ADVICE: Beginning authors often ask me should they write the entire trilogy before trying to sell it. My advice has always been that the first book SHOULD be able to stand on it's own, but MAY be connected to a trilogy. Unless you're writing an story that specifically needs three books (often in fantasy, ala LORD OF THE RINGS, or some romantic suspense trilogies that are created with an over-arcing mystery) then it's best when starting out to give an agent (and publisher!) the OPTION. Why? Because you're positioning yourself to be able to do A or B. If a publisher loves your book but is over-inventoried in trilogies, they may not want to invest in another trilogy. By giving them the option, it shows that your book CAN stand alone. Publishers love connecting books . . . when they succeed. So introducing the hunky brother as a secondary character is always good! But if you write all three books, then you're tying the hands of the publisher. And if your first book isn't ready for publication, then you've spent a lot of time developing a story that isn't going to go anywhere and will likely change dramatically as you learn.
So if you have a trilogy idea (or a series), my advice is to write the first book and a 1-2 pager for the next two books in the trilogy/series. For published authors, this can be cut to a proposal (three chapters/synopsis) plus one page on future books; or even a synopsis for the first book and a paragraph for future books. Most unpublished writers they want the complete manuscript.
This is a novel that stands by itself--it does not have a major character who recurs in each book (series) nor a trilogy with recurring characters. Each book is a complete story that is done when you're finished. Thrillers and suspense novels and the "big" books are often stand-alones. Though Lisa Gardner has recurring characters, her books are more stand alone than a loosely connected series. Nora Roberts big hardcover that comes out every July is usually a stand alone. Many of the big romance writers (Susan Elizabeth Phillips, Nora Roberts) write stand alones; you'll often hear about a "stand-alone" written by an author who also has a series (Lisa Scottoline's LOOK AGAIN or Michael Connelly's THE LINCOLN LAWYER--even though he brought Mickey Haller back in a subsequent Harry Bosch book).
I wrote THE PREY as a stand-alone; my publisher when they bought it asked me to connect it into a trilogy. Stand-alones are usually hardcover, which is why you find more of them in thrillers/suspense than in romance. Romance novels are predominately published in mass market (and now trade.) There are many reasons for this, but the primary reason is that romance readers read ALOT more than other genre readers (and they read across genre more than other genre readers) and they can't afford to buy hardcovers en masse. Where the big thriller readers will read a couple books a year, and thus happy to buy 4-6 hardcovers because they only read that many books a year, romance readers read 4-6 books A MONTH.
Okay, any questions on this? Anything else?
I'll stick around for the rest of the week since last week got away from me and I didn't post when I wanted to!