You no doubt have a number of links on your website that point to your many working partners, suppliers, customers or other schemes that you're running. If you configure these links to open in a new window without clear instruction, it can be confusing to users with visual impairments, who are not necessarily aware of the new browser window opening. To combat this, many designers operate a system that notifies users when they are opening a new browser window through the link text: this is a horrendous half-measure that ignores the root causes of the issue and, moreover, fails to acknowledge the complete myth that created the "Opens In A New Window" concept.
Opening in a new browser keeps your website open so that users can return to it once they're done visiting another site.
I don't get to use this word online a lot, so I'm indulging myself: POPPYCOCK. Simply writing "Opens In A New Window" to alert visually impaired users is a giant waste of time - you shouldn't ever have to open in a new window anyway!
Studies undertaken by a big usability organisation, the Nielsen Norman Group (published in Prioritizing Web Usability) demonstrate that users view the Web as a combined whole and not a series of self-contained sites. Users will often visit and discard sites, if they feel that your website doesn't have the information they need: in instances such as this, having your site open in another window is just annoying.
Worse still, every new browser window restarts the browsing history so, if the user closes your window to reduce the items on their taskbar, they can't return to your website using the "Back" button. In many instances, the "Back" button on any particular browser is king: it allows the user to retrace their steps when they feel they have made a wrong turn. Breaking it is one of the worst sins a designer can commit (and a good example of why Flash should never be used for websites).
DON'T DO IT! A number of our articles have stated that this is a fundamental practice of good Web design. You can't trap your users in the site and, even by trying, you're going to alienate a number of users who don't appreciate it. The Internet is an open medium and you will be able to gain more through an user-friendly website than you ever will through attempting to box your users in - don't make your website a chore for your customers!