A plot, as E. M. Forster pointed out in his landmark book Aspects of a Novel, is not merely a sequence of events: “‘The king died and then the queen died’ is a story. ‘The king died and then the queen died of grief’ is a plot.” Plot is a “narrative of events, the emphasis falling on causality.” In other words, a plot is a series of interconnected, not random events. Forster went on, “If it is in a story we say ‘and then?’ If it is in a plot we say ‘why?’”
In every plot, there are three key elements:
Obstacles to be overcome
Different genres of fiction have their own plot elements, as well. For example, in a mystery novel the plot moves the main character from puzzle (like Who Killed Roger Ackroyd?) to solution, taking him (and the reader) from confusion to knowledge. The main question driving the plot is usually something such as: Will the sleuth identify the killer before he can kill again?
Plot is what characters do to adapt to whatever situation they are in. Just like real people, characters react when something happens that alters their status quo.
Compare this to the basic elements of plot in a romance novel. In a romance, the plot drives two characters apart and then brings them together, taking them from unhappiness to happiness. The main question driving the plot of a romance novel is usually something such as: Will Bill and Mary find true love together before it's too late?
The Main Plot
The main plot is the story that forms the backbone of a novel. It starts at the beginning of the book and is finished by the end.
In a fantasy novel, it could be the story of a character's quest to recover some sacred object. In a thriller novel, it could be the story of how a character infiltrates al Qaeda and prevents a massive terrorist attack. In a literary novel, it could be the story of how a character runs away from an abusive home and finds the strength to return and face her abuser. Regardless of the type of novel, the main plot concerns the protagonist's struggle to reach a goal.
Subplots are the smaller series of interconnected events that run through a novel, often interwoven with the main plot. These may be plots that belong to secondary characters, or simply smaller stories unfolding at the same time as the larger one.
For example, while tracking down a killer, your protagonist's apartment might get infested with bedbugs so she's forced to move in with her sister whom she detests. Tracking down the killer remains the main plot, but the battle of the bedbugs and sibling rivalry are subplots that provide comic relief.
Subplots work best when they're connected to the main plot. So, for example, a subplot that involved the sisters finally resolving past issues would be particularly effective if that resolution helps the protagonist gain some insight into the identity of the murderer.