3D Secure is an online security feature designed to ensure that the person placing an order online is in fact the legitimate card holder, rather than someone who may have stolen or cloned the card. It's a bit like "Chip and PIN" for the web.

The two main implementations of 3D Secure are 'Verified by Visa' and 'Mastercard SecureCode' - it's the same system just different versions. Some banks may also use this system under another guise, Lloyds TSB for instance implement 3D Secure under the name 'ClickSafe'.

If you are not redirected to your bank on the final payment screen. You may need to create 3D Secure 'account' via your banks website. Below is some helpful links

Nationwide 3D secure webpage - http://www.nationwide.co.uk/vbv/

Natwest 3D secure webpage -  https://www.natwestsecure.com/natwest/registration/welcome.jsp

Barclays 3D secure webpage - https://verifiedbyvisa.barclays.co.uk/barclays/registration/welcome.jsp?partner=debit.visa

Halifax 3D secure webpage - http://www.halifax.co.uk/creditcards/secure_home.asp

HSBC 3D secure webpage - http://www.hsbc.co.uk/1/2/popups/verified-by-visa-information

Lloyds TSB 3D secure webapge - http://www.lloydstsb.com/clicksafe.asp

3D secure explained by Visa -  http://www.visaeurope.com/en/cardholders/verified_by_visa.aspx

3D secure explained by Mastercard - http://www.mastercard.us/support/securecode.html