When I write a story that I lose interest in (plot, characters) I don't complete it and throw it away.
I'm not quite as extreme, as I rarely throw anything in the virtual trash, but I've found that, if I don't act on an idea without a certain number of months, my ideas for the story harden, so what should be a fluid story that essentially tells itself, I become locked into a rigid approach to the story. Thus, anything I don't address in three to six months, I generally consider dead ends. If I don't get to it in that amount of time, I assume the story premise wasn't really that intriguing if it didn't fight stronger to be written. However, as Ernest attests, that's just me, as he actively keeps multiple stories going at the same time, often spanning years between each individual project.
For many of us, mainly those who write 'complete' stories before posting, we write the story to it's natural ending, and write to the conclusion (i.e. we don't keep cranking out chapters to keep the story going). Since we start out having a clear idea where the story will conclude, it's more or less a straight line from the beginning to the end, with multiple surprises for us along the way we never anticipated.
To answer your question, which plays into my point, a serial is a story (on SOL) while continues, chapter after chapter, in an ongoing effort for some time. A series is a collection of concrete 'books' (each a complete story in and of itself), which continue a given story line.
However, having a clear end doesn't mean the story has to stop there. Often, I'll write a given story, knowing where it'll end, but after finishing the story and posting it, several months later, when I'm not even considering the story anymore, I'll suddenly get an insight into an entirely new aspect of the story, which gives me a way to continue the story while making it into a completely new tale. It's at that point that I add a book to an existing series.
When I first wrote Love and Family During the Great Death, it was intended to be a 'one and done' story. I was interested in writing an apocalyptic tale
that address how the individual involved though about the deaths of their families and loved ones. Once completed, even though the two main characters did survive, I wasn't terribly interested in writing 'yet another' post-apocalyptic tale, thinking I had nothing to make it markedly different than the many already out. However, months later, I realized that how those people dealt with their losses impacted the sequel, as instead of fighting over the spoils, everyone simply wanted to be left alone in their grief, even as my main characters realized there were future catastrophes on the horizon, and they had to organize the survivors in order to prevent even more deaths.
It'a akin to the difference between writing a 'day in the life' story, where you simply focus on what happens from one moment to the next, and writing an 'episodic' story, which jumps from one specific episode to another. Rather than covering everything that happens in any given day, each chapter deals with a specific event, whether that event happens in a single afternoon, or covers multiple days.
Once you start looking at episodic stories, it's much easier to control the story, as you ONLY include those items that are central to the plot. It also becomes much easier to control the length of your stories when you think them that way.
In a series, each book is a complete 'story', a string of episodes which involve a specific conflict, but which doesn't resolve the ultimate conflict addressed by the entire series until the final resolution of the entire series.
And once you resolve the essential conflict in a series, there's really nothing else to add beyond a few additional chapters which aren't going anywhere in particular.
Finally, in a universe, you have a string of complete stories which all occur within a given fictional universe. Thus I could have had multiple authors writing in my Great Death universe, by addressing what happens to the survivors in different nations across the globe in the same basic timespan.