have seen in some websites but dont know how people could do it.

In an email subscription form, a line of "please enter your email here" is preset in the input area. When you click in the input area the line disappear and give the space for you to type your email address.

This will make it more convenient for user and save time deleting the line before entering the email

Thanks for your support

Here's the example of how this effect can be accomplished:
http://www.templatemania.com/example...Form/index.cfm

Just click "View Source", to see HTML and Javascript.