Sometimes it is desirable for a link to open in a new tab or window, so that the page with the link does not get replaced. Dreamweaver provides a Target field in the Properties Inspector for this purpose. This Target field remains inactive until the Link field is filled in, at which point it gives you the options shown in the screen shot below:
The _parent, _self, and _top values apply to framesets and should not be used (unless you are designing with framesets, which you probably shouldn't be).
The _blank and new values will both direct the linked page to open in a new window. The difference is that all links that target _blank will spawn new windows, whereas a link targeting new will open a new window and name it "new". As long as that window remains open, all future links targeting new will open in that same window. In fact, new is an arbitrary target name. You will get the same behavior with any target name other than the four reserved names: _blank, _parent, _self, and _top.
Be careful not to overuse targetting new windows. A best practice in web design is to give users what they expect and users generally expect links to replace the current page. They also know that they can get back to the original page using the browser's Back button. However, when creating a link that opens in a new window, the user will not be able to use the Back button in that new window to get back to your page. Reserve the open a new window for links that take the user away from your site, so any links to other sites work well in a new window as your site will still be available when they close that window.