multiple wordpress blogs

Sounds like you need WordPress MU.

no … it is not multi user what i am talking about
it is multiple blogs controlled by single interface

See if this would help: Installing Multiple Blogs

let me explain again

i have a site
with two categories
1. news
2. articles
i want to use wordpresss for both of these cat and i want my users to be able to register once and able to post on these both diff cat ( customized form of blogs)

I don’t think you need a separate blog for each category. What you need is:

1. One blog with two categories.
2. Plugin, such as Userextra, which will allow you to give your users access to one or the other category (or both).

Is this what you need?