What Exactly are Serials, Sequels and Spinoffs?

There are 3 main types of series: serials, sequels and spinoffs and each one is slightly different and well worth a mention.
A serial is where the same main character is featured in each book, but each book can stand alone. Most mystery/suspense/action/adventure series fall into this category. While you learn more about the character as the series progresses, the stories themselves are mostly episodic. Examples include The Hardy Boys and Indiana Jones Adventures.
Serials can also be released in smaller, sequential versions for readers to enjoy, such as in magazines. Originally, the Sherlock Holms stories were written for serialisation in The Strand magazine. The Count of Monte Cristo had 139 instalments. Wattpad is one site that serialisesaspiring writers’ works if anyone is interested enough to give them a go.
Sequelshave a finite number of books where the plot is introduced in the first book and concludes with the last. Many fantasy and science fiction series fall into this category. While individual books can be read as standalones, readers get more out of the narrative when they start with the first book and follow the series in order.
Examples of sequels are the Harry Potter books, the Shannara and Twilight series.
Spinoffs take a minor character, setting or plotline from the original standalone and develop it. In many cases, the series characters are a specific team engaged in a heroic profession. Spinoffs work well for romance writers who need a fresh couple for each book’s romance, although it also works well in the fantasy genre. The YA science fiction I Am Number Four series has quite a few spinoffs.
Writing a series has several advantages. For instance, a series allows readers and writers to get to know extraordinary characters and explore the created world in detail. Strong characters make a series memorable for readers who witness their evolving traits, conflicts and growing relationships through the story. They also witness their growth and change with all that’s happening around them. 
Another advantage is that you, as the writer, already have your idea, characters, setting and you know your story inside out. You have the basis to keep you writing going for years and therefore you can write faster with well-established characters and worlds. This means the more books you have, the more sales you are likely to make. The more you sell, the more confident and grounded you become, creating a loyal fan base. Writing that series will be a winning situation for you and your readers.