TRANSITIONS are used whenever there is a change in the story, say at the beginning of a chapter, when we move from one place and time to the next, or from one point of view to another. We use transitions to orient the reader as to where, and when, they are now, and who they are with. Example:
The stalker phoned me for the first time early on a Saturday morning. The ringing of my cell phone woke me.
INTERIOR MONOLOGUE is the character’s thoughts. When we read interior monologue, we’re inside the character's head, mind-reading. Example:
I felt sorry as soon as the words came out of my mouth.
Again, all these modes of storytelling, or building blocks of fiction, combine to form scenes. SCENES move the story along and hold the readers interest. Things happen in front of the reader, in real time (the "now" of the story). When handled well, the reader feels they are right there with the protagonist.
FLASHBACKS are special kinds of scenes that take the reader back in time in relation to the “now” of the narrative. They are the character’s memories.
In the past, I've brought my son's building blocks into my classes to demonstrate these elements in fiction. Think of a scene (action, dialogue) as a block on wheels: it moves the story forward. Narrative summary or transition is a bridge that connects two blocks of scenes (it moves from one place and time to the next). Description is a block that builds up from the scene: it builds the picture of landscape or character in the reader's mind. Lastly there are the little blocks of exposition, or explanation. They are useful and necessary, but should be used sparingly, as that particular block has no wheels. Narrative drive comes to a stop when the author or a character starts explaining (or preaching, lecturing or teaching).
One way to really get a handle on these building blocks is to check out how they work within novels you admire (or how they don’t work in poorly written novels). Pay special attention to exposition. Generally apprentice writers overuse exposition. I'll often highlight exposition in yellow within a student manuscript, to illustrate just how much exposition is in use. Do it for your own writing and you may be surprised.