Pop-ups are additional browser windows that are opened by a website. Often simply annoying, as they're frequently used to push advertisements at the viewer, pop-ups can also be harmful. Pop-ups can contain malicious links or unsuitable material. 

Most browsers these days come with filters that automatically block pop-ups, and controls so you can allow pop-ups from benign sites. Some webmail programs still use them, for example. 

Should you ever need to block pop-ups yourself, here's how in each of the three main Windows web browsers. 

Internet Explorer

Go to Tools > Pop-up blocker and select Turn on Pop-up blocker. Click Yes. 

If the option isn't available, pop-up blocking is active. 

To allow pop-ups from a specific site, go to Tools > Pop-up blocker > Pop-up blocker settings. Type the address of the site in the box and click Add > Close. 

Firefox

Go to Tools and select Options. In the dialog box that appears, click on the Content tab. 

Make sure there's a tick next to 'Block popup windows'. 

Click on the Exceptions button next to the Block popup option and type the address of a website that you wish to allow pop-ups for. Click Allow > Close > OK to finish. 

Google Chrome

Click the spanner icon, then choose Options and select the Under the Bonnet tab. 

Click the Content Settings button and highlight the Pop-ups category from the left-hand list. Make sure there's a tick next to 'Do not allow any site to show pop-ups (recommended)'.

Use the exceptions button for sites you use on a regular basis that rely on pop-ups for other, more innocent reasons. For example, some webmail packages use pop-ups for composing new messages.