There are two things that define every story: Its theme and its mood. Everyone can recognise these two traits in a any story, even if only subconsciously: If two stories have the exact same mood and theme, no matter the difference in characters, settings and other details you will recognise that one of them is a piece of plagiarism. Thus it is important to be aware of what kind of moods and thems you can use, not only for the sake of avoiding plagiarism, but also because it gives the writer a broader view on what can be written, and how.
What is Mood?
Mood is the "feel" of the story, the emotion that it conveys to the reader through the language used by the writer. If you read a love-story or a thriller, you will see that the descriptions, the emotions described and the characters have a different "feel." The same building can be described in as many ways as there are emotions. In a love-story, the Royal Palace in Gamla Stan will be described as a romantic, glorious place where the two main characters meet for the first time. In a horror-story it will be a dark, brooding place that looms over the characters with dark, empty windows and bleak, fading colors.
The mood is by no way controlled by what theme you chose: A love-story does not necessarily have to be all pink and flowers, it can be about lovers that live in fear of someone or something, or it could be a comical love-story, like many movies. It could also be a thriller with a romantic mood, or an adventure story with a dramatic mood. The most interesting stories have a theme and a mood that do not normally mix.
So, when you write a story, it is a good idea to have an idea about what kind of mood you will want to convey to the readers.
More often than not, you will want to open the story with a hint of the mood. Either by the narrator telling a little about the results of the story, without giving the reader any hints about why or what has actually happened (this trick is called foreshadowing), or by letting the first descriptions of a character, secondary character or scene have a strong feel of the mood. A scary man watching the main character, a romantic place, or the main character dressed in a silly, funny manner.
Once you hace done this, you need to occassionally add a few details or hints to the mood. A couple watching the birds in a parc, a black raven that glares at someone, something that confirms the mood you want to convey.
Examples of Mood
There are many different moods available, and the ones below are only examples:
Brooding: The characters in the story are haunted by dark shadows and nothing is really safe or reliable. There is a great evil somewhere, but it will not reveal itself until the very end of the story, if even then.
Romantic: Everything is beautiful and special, every word carries special meaning to the characters. Loneliness and ugliness are only around for when the characters have doubts or feel betrayed, but otherwise all things should be bright and unique.
Mystierious: Things are unclear, and everything important in the story tries to remain hidden. Antagonists try not to show their real motives, no-one knows what they are after. The streets are dimmed by fog, and the main characters rarely understand all that happens around them.
Excitement: Lots of action, little space for description of personalities or unimportant places. Lots of things happen, often at the same time. The characters rarely get a chance to rest.
Dank: This mood is a foul and rancid one. People are corrupt and degenerate, grotesque and bizarre. This is a mood for dystopic stories.
Idyllic: This is a mood of peace and tranquility, where everything is as it should be. Even though this mood can get very boring in and of itself, it is a beautiful "trap" to change into other moods, especially more freightening ones!
Humorous: Things are absurd and silly, people say or do idiotic things, or react in the right way but in a very odd manner. The hard thing here is not to overdo it, so that the theme suffers, but to keep things at level where you can still tell the story.
Eerie: This is the mood of the unreal. Things and events seem like in a dream, or at least a strange daymare.
Madness: In this mood, things are out of control. There is no ryme or reason, things happen randomly and never when they are expected. This mood carries a sense of delirium and hallucinations.