Deja vu is the idea that you have experienced or previously seen something that you actually have not. Deja vu is French for "already seen." Most people, including myself, have experienced(or thought we have) deja vu at one point in their lives. Deja vu experiences can seem can seem extremely familiar which is why people believe the idea of deja vu to be real. It is a very popular idea in our society and culture. There was even a movie based on the idea of deja vu(it is actually called deja vu).
While some deja vu experiences seem unmistakeable, there are many explanations to it. Deja vu illusions are more likely to be reported by people who remember their dreams, travel often, are young, have liberal religious and political beliefs, a college education, and a high income. An excess amount of the neurotransmitter dopamine in the temporal lobes is known to play a role in the occurrences of deja vu. Seizures also play a large role. The right temporal lobe is responsible for feelings of familiarity so people who have seizures in the right temporal lobe can experience deja vu before a seizure. I think that the most reasonable explanation is that people don't consciously remember an experience, setting, or event. it is easy for us to get distracted and forget what we are seeing. We can forget that we have been to a certain place and then return(maybe years later) and think that the familiarity we feel is deja vu when we have actually been there previously.