When using Safari, the default is generally that it uses Tabs instead of a new window for each new page you want to open.  This is usually a great thing in fact, as it keeps everything much more organised on your screen and allows you to make the most of your valuable screen real estate.  A typical tabbed browser window will look something like this:

tabbed-browsing

Notice that there is one tab for the "Apple" page and another tab for the "Facebook" page in this case.  You can open as many tabs as you want and they will squeeze in to fit along the top.  This lets you quickly move from one page to another without having different windows all hidden behind each other and it's a very useful feature.

However, sometimes you'll actually want to have one of those tabs in a separate window.  Maybe you want to compare two web pages side by side, which is difficult to achieve using tabbed browsing.  You have two options to accomplish this.  One, of course, is to open a new window from the File menu and then open the address of the page you want in that window.  But what if you already have the page you want open, but it's in a tab already?  There's a great little shortcut to get that tab out into its own window.  You can either grab the tab by its title and drag it out of the window that's it's currently in, which will move it to its own window, or you can right-click the title of the tab and from the context menu that appears, choose the option Move Tab to New Window.

tab-to-new-window

You can then position your two windows side-by-side if you wish.  So as you can see, it's a pretty simple task to move a tab to a new window in Safari once you know how!