Termabulan is mistaking web content writing for webpage production. Arif, the content of the web pages are what your client (or you, if you are doing for yourself), want the audience to see, know, feel, do. So if you have a message to get across, how effectively does the page you have created do it?
This comes down to a combination of good catchy writing skills - you need to get viewer's attention quickly and hold it till they have got your message - and an eye for design. There are tips you can follow regarding how best to lay out text and images to catch the viewer quickly.
It also helps to be abe to advise clients about alternative ways of communicating messages eg through audio clips, video clips, movies made in Flash etc, or by including games and interaction.
As a web designer you may get contracted by someone to do work that you know won't work well - typically, pages and pages of writing with a few mugshots of the director thrown in - and you need to decide how best to advise them that they maybe need to rethink their approach.
And you also need to combine this with an awareness of accessibility issues ie coloration of text / background / text size / how interactive elements are included so that you are meeting the requirements of all potential viewers and not excluding viewers with disabilities.
You need to work closely with a web designer (which might be yourself) to ensure the pages you create are working well in a variety of browsers - IE, safari. firefox, opera - and will rescale for many different devices - iphone, umpc, pc etc
That makes you a good web content writer.