What is a scene? It is a section of drama, like a brick is a section of a wall or a step is a section of a staircase, without which the entire structure, or story, would fall. And just like any brick or step, it has a beginning, a middle, and an end.
Generally, it begins with action, and it begins at a point that 1)makes sense to your story, and 2)takes place sometime after the prior scene it most relates to. For instance, if you have two Point of View Characters (POV) and scenes A, B, & C are from POV #1, followed by scene D which is POV #2, your next POV #1 scene would come after and relate to scene C.
This leads me to two other elements each scene must have. First, it must have a purpose; i.e., a reason for taking up your story's precious time and space. If it doesn't, you will at the least, lose your reader's attention, and at the most, fail to convey your story's message. One important way to create this focus is to make sure your POV has a goal to accomplish in that scene. Another is to make sure the scene, itself, has a goal. For example, is the scene's purpose to introduce the opposition? To hide a clue? Show that your POV is falling in love? If you can't answer these questions, chances are, you need to eliminate the scene.
Second, scenes must be "causal." In other words, scenes must build upon each other in a cause and effect manner until they create the complete whole. Another author described it this way: scene A leads to scene B, and then to scene C. And later scenes F and G cannot happen if scene B was not thoroughly developed.
Another vital characterisitic of a scene (the majority of the middle section) is that it must contain dramatic conflict. Sometimes those conflicts are great battles between nations or even two people, but other times, that conflict--the sought after "tension" of your story--is created simply by showing someone or something standing in the way of your POV's scene goal or story goal.
Finally, a scene ends when your POV either reaches (or more likely, fails) to accomplish his goal and creates a new goal, thereby propelling your story (and reader) forward.
So that, in a nutshell (or a toolbox, in this case), is what a scene is. Much of my understanding of this structure comes not only from my own experience, but also from Rachel Friedman Ballon's book, "Blueprint for Writing." I have also found good points in Randy Ingermanson's article, "Writing the Perfect Scene."
You might want to give them a try.