As you read through this guide to genetic engineering, you'll find links like the one you just clicked to bring up this little window. Each of these popup windows gives you a bit more information about something important, but they don't have to interrupt your reading unless you want them to. You can drag these little popups about and also resize them. In some, you'll find hyperlinks which interconnect between the popups. If you've clicked one of these links and want to come back to the popup you started from, just click anywhere in the popup with your right mouse button. Then click 'Back' in the mouse's menu and you jump back again.

In some of these popups, you'll also notice numbers like this: [3]. If you scroll to the bottom of the window, you'll see that these simply tell you where I found out about things. (Dates are Day/Month/Year followed by the page number so you can find the original information if you want to.) Some have links to special pages on other sites where you can find more about something. By clicking those links, you won't lose my guide because a new window will open.
Hope you find all this helpful. Good luck!