Series or Single?

Okay.  So you have decided to write a book.  And publish it.  You're brainstorming ideas.  Before you start outlining, you need to ask yourself the question: Am I writing a series or a single book?

[UPDATED THOUGHTS: I would highly advise honing your early story craft on either a single or a trilogy. You learn a ton, and then move on. If your first series is, ahem, six books long, when you are done, books 4, 5, and 6 will obviously be stronger works, but you can't send readers to them. They have to go through your first efforts. Find your footing, and then tackle the bigger stuff, if you want to!]

Series v. Singles
Readers like to get to know characters.  They like to come back and learn more about the world they have come to know.  They are also statistically more likely to invest their time reading a book that they know has more coming.

Single books can work, too.  You typically need a very strong theme.  The possibility of adding to it later doesn't hurt either. These let you learn to finish telling a story, and to clean it up.

What Passes for a Series?

• A group of books that each tell a consecutive part of one big story.  Think Lord of the Rings.
• Books that use the same characters, but each tells a complete, stand-alone story.  Think The Magic Treehouse series, or a television show like NCIS.  This works well for mystery or detective works.
• Books set in the same universe.  If you did a steampunk retelling of Cinderella and then used the same style to do The Princess and the Pea, you make this into a series.  Characters can even intertwine (for example, Ruby West is actually cousins with Alexi Daniels).

​Each of these can work really well.  You just need to take some time to decide what's right for your writing.  It's wise to set up the type of series you enjoy, so your readers can come back when they want more! I would remind you to make sure you practice finishing stories first!