We are creating a business directory. Is it possible to populate the
information which is available on the websites of companies
directly into our database without taking the consent of respective
company..Also what about the information such as AAA diamond rating of
hotels or D&B number, stock symbols ..What does the copyright law has
to say about these issues..

Copyright only protects items which are sufficiently creative and
original to merit copyright protection.  Mere assemblages of unadorned
facts typically do not qualify for copyright protection.  For example,
if I place on my web page a list of all 50 United States capitals,
typed in 12 point Courier font, I would have a very difficult time
alleging copyright infringement if someone copied that list off my
site and placed it on their site.  It is just a list of facts.

But this is a continuum.  The more creativity I place into the
selection and arrangement of the facts, the more likely it is that I
have added enough creativity and originality to make it protectible. 
For example, if I take my list and alphabetize it and add colors and
cool fonts and graphical logos, I may have added enough creativity to
render that work protectible.  But even if I have, remember that the
underlying base facts?the names of the state capitals?remain
unprotected facts.

Another example: the King James Bible is in the public domain.  But if
I pull out selected verses and arrange them in chronological order and
add commentaries, that work is protectible.  The underlying verses,
however, are not.

So, to answer your question with a question, is the data you wish to
use raw unadorned plain old facts, or is it data that someone took
some time and care to select and arrange?  You are in a gray area with
stock symbols and diamond ratings.  Stock symbols likely cannot be
copyrighted.  Diamond ratings very likely can be and are.

Thanks for your comment but let me clarify what I am looking for..
I want to list companies say carpet services in the area in my
directory (with their website, size of the company, and contact
information) Do I need their permission to list them or
not..Essentially, I want to create a directory similar to yahoo
directory but not by crawling the web but by manually entering
information. I am curious for listing what permissions I need and for
what kind of data..

From where will you be getting your information? If you are going to
walk round your neighbourhood writing down all the businesses whose
shopfronts you can see, ok. But if you are compiling your directory by
copying the information in an existing directory (whether automated,
copy-and-paste, or retyping from a printed directory) you are
infringing the original directory compiler's copyright.

And directory publishers salt their directories with imaginary
entries, minor mis-spellings, etc, to assist them in detecting theft
of their data.

To quote from one directory publisher's terms and conditions:

Except where otherwise noted, Yell owns legally and beneficially all
the intellectual property rights in the content of all the Yell.com
web pages (including the design, text, graphics and arrangement
thereof) and in the software used therein. ... Yell takes active
measures to ensure that no unauthorised use or breach of its
intellectual property rights is committed. ... For the avoidance of
doubt and without limitation, you are expressly prohibited from:  ...
merging the Data with any other data; selling or trading in materials
copied from the Data; using or redistributing the Data for the
purposes of compiling databases, lists or directories ...
http://www.yell.com/legal/home.html

But you do not need permission from business simply to include them in
a directory. You may want to get their permission to reduce the risk
that one of them may consider that by including them in your directory
you are implying some sort of connection with or endorsement of your
directory by them.

Owain