What is parallel universe?
It is a nonsensical concept that has no basis in physics
EDIT: if you do not know how to use GOOGLE, you really should learn.
Physics Forums is a great place to answer specific questions, however if you wish to know the general overview of a subject then your best bet would be to hit up Wikipedia or another source of information. Imagine if someone kept asking you "What is X" when they had an encyclopedia right there next to them!!
Parallel universes is a concept that originated in quantum physics, then reemerged in string theory. The basic idea is built on the 'many worlds' interpretation, where all possible outcomes of a quantum event must occur. The observational issue is we can confirm only a single outcome. That renders it unconfirmable by any known test, hence, it has little scientific merit.
friends first and second replies was expected....
the thing is, if I would have not know how to use google then probably I would have not physics forum and what parallel universe....so don't tell me that....anyway I was asking some specific things....
No you weren't. You asked nothing specific at all. Even your original question is too broad, as there are several possibilities on what you could be asking about. Searching for parallel universe on wikipedia brings up the following page: http://en.wikipedia.org/wiki/Parallel_universe
As you can see there are two options under Science and several in other areas. I recommend figuring out which one you are asking about and reading the relevant article on wiki.
Separate names with a comma.