what is it, and how does it happen?

Deja Vu is a feeling that you've been some where,seen something before, then it happens just as you sort of remember it .
No one knowa but those of who have had it know it when we see it again.

Déjà vu (pronounced /ˈdeɪʒɑː ˈvuː/ (help·info); French /deʒa vy/ (help·info) ''already seen''; also called ''paramnesia,'' from Greek παρα ''para,'' ''near'' + μνήμη ''mnēmē,'' ''memory'') is the experience of feeling sure that one has witnessed or experienced a new situation previously (an individual feels as though an event has already happened or has happened in the near past). The term was coined by a French psychic researcher, Émile Boirac (1851–1917) in his book ''L'Avenir des sciences psychiques'' (''The Future of Psychic Sciences''), which expanded upon an essay he wrote while an undergraduate. The experience of déjà vu is usually accompanied by a compelling sense of familiarity, & also a sense of ''eeriness,'' ''strangeness,'' or ''weirdness,'' The ''previous'' experience is most frequently attributed to a dream, although in some cases there is a firm sense that the experience ''genuinely happened'' in the past.