"I always have a basic plot outline, but I like to leave some things to be decided while I write." ~J. K. Rowling
Although it's pretty obvious, I'll take a moment to define plot. Plot is the what happens in a story and the sequence in which it occurs. Some writers like to use outlines to plot their novels before they start, others do best just jumping in and deciding how things will happen as they go. Either way, a good plot needs certain elements.
Character - Of course, if anything of interest is going to happen, there needs to be someone for it to happen to, or to make it happen. Characters should be realistic representations of people. Make the readers care about this individual.
The hook - This is where you grab your readers and make them want to keep reading. It could be your protagonist making a life changing decision, or being forced into a situation he would have never chosen.
Conflict - The character runs into trouble. He wants something so badly he's willing to do anything to get it, but there's something or someone in his way.
Stakes - The higher the stakes, the more your readers will care. If you're protagonist wants something badly but everything will be just as before whether he gets it or not, the stakes aren't high enough.
Subplots - In real life, we never have just one thing going on. Add in family life, job life, or whatever else your character has going on. Just make sure to tie it into the main plot so it's not just there to look pretty. It has to have a real purpose for the story. If you could cut the subplot without the story suffering, it's not a good subplot and should either be cut or worked into the story in a meaningful way.
Climax - This is what it's all been building up to. All the conflicts, stakes and character struggles come to a peak and play out in a grand finale.
Resolution - This is where you tie up all the loose ends.