Users expect new pages to open on the same window on which they were currently working. By making pages to open on separate windows, even those outside your website, you are going against their expectations.

• Don’t make pages to open on separate windows, except when they are PDF and similar documents.
• Think about your older and visually challenged audiences. They can’t manage many windows.

Web developers and designers are often afraid of opening the links on the same windows for the fear of losing the visitors. But that is a lost cause in any case. If people want to leave, they will. You can do nothing about it. And if they want to come back, they can always use the back button (back button is the single most used button on the web).

There are several usability problems that come from opening pages in different windows. The Usability guru, Jakob Nielsen, provides a detailed study of the associated problems in his book Prioritizing Web Usability.