Hmm...deja-vus. This is rough, uneducated and definitely not based on any research. I'm just a person who became self aware (during the waking hour and in my dreams), and through this awareness, was able to explore what was happening to me.
You are four parts: your mind, your body, your spirit (the eternal part of you (ancient)) and your soul (your essence/aura for this life).
Specifically, you might ask, what is the difference between the spirit and the soul? Here's your answer: Your soul is your astral body and your spirit is your "higher" mind. It works in the same way your brain (lower thoughts) and body work together. It is essentially the same type of relationship.
What most people don't know is that they travel almost every night when they sleep. The people that realize that they are having this experience often explain it as being asleep in their room, unable to move, feeling frozen with the ability to see everything happening in the room. The reason these people can't move their physical bodies is because they're trying to move their physical bodies (which are sleep) and they're actually outside their bodies (soul) viewing everthing with the spirit and translating with their brain. For what purpose? Well, because they are their spirits first and it's the nature of being a spirit...to travel (with the soul body, of course). That's a most important thing to know. This is what most people remember about the experience, but there is so much more. It this state, as you go deeper, either you: Connect with God, time travel, travel to local or foreign lands to witness events, visit people on this plane or on the other side, etc, etc..Who knows the reasons? I suspect it's to help us overall with life and all of its adversities and trials; to help us achieve, overcome; to warn, if neccessary.
As a result, it's important to realize that the higher mind (spirit) knows almost everything and in most cases, it has seen your future. It's hard to believe, but just because your spirit knows something, it doesn't mean your brain knows. However, when the brain does see something (during the waking hour), it cross-references the scene with the spirit instantaneously, almost like a computer. When it does, sometimes there is recognition. As a result, you experience deja-vus. That's where they come from; your spirit knows the future and your brain, during cross referencing, recognizes what the spirit already knows. Along these lines you must understand that dreams are simply the lower mind's (brain) attempt to translate what the spirit (higher mind) is seeing or experiencing. This is the reason that dreams sometime make perfect sense, but they have weird elements to them (blue pigs flying, pink clouds, etc.). That is because the brain is creating images to interpret what spirit knows and because it is lower, it doesn't always get it right. Actually, it gets it right, but the images are definitely subject to interpretation.
Also of importance: Your spirit is eternal and is VERY wise. Life is a journey and experiences contribute to your growth. Your own spirit will keep things away from you. Let me rephrase that: you'll keep details away from yourself until you are ready to accept them. This is probably the hardest thing for most people to accept. Most people would hardly believe that they know the future but would still allow themselves to go through certain things and not prevent them. As previously stated, your own spirit is wise and knows what will add to your existence down the road.