Some of the following information has been sourced from Lorelle on WordPress:
http://lorelle.wordpress.com/2005/09/15/wordpress-power-user-features-just-press-it/
Press This is a bookmarklet: a little application that runs in your web browser (Internet Explorer, Firefox, Safari) and lets you grab bits of websites and publish them on your website.
You need to save the Press This application to your bookmarks or favorites before you can use it.
Then, when you are visiting a site you want to post about on your website, just click the Press This link in your favorites or bookmarks and a window will pop up with either your website's login or, if you are already logged in, a condensed version of the Write Post screen. Inside will be the title filled out with the title of the website you are visiting (if there is one) and a link in the Editing text area. You just fill in the text, mark the categories, then save it as a draft or private or publish it.
The Press This screen looks like this:
NB: Before using Press This you should consult with your web developer or IT department and you should seek legal advice regarding using content from other websites on your own site.