The basic structural building block of any novel is a character who wants something and acts to get it.
The account of what happens when that character acts to get what he wants is the plot, if the character is the main character and the objective is the main storyline. If it’s a secondary character, or a minor part of the main character’s story, then you’ve got a subplot.
Each plot or subplot has three parts: a beginning, in which we see the character and his problem or desire, a middle, in which the character acts to achieve what he wants, and an end, in which we see how it turns out.
How do you know which story thread is the plot and which is the subplot? The nature of the most interesting story thread tells you what genre of novel you’re reading. Twilight has a romantic relationship storyline, and so does The Lord of the Rings. In Twilight, the Bella-Edward agonizing is the part the readers care about most and that fills most of the space, so Twilight is a romance novel. In LOTR, the Aragorn-Arwen relationship is understated to the point of near-invisibility, so that book is… not a romance.
And don’t try telling me Frodo and Sam have a romantic relationship, ’cause it just ain’t true.