In many countries, EMV is used to prevent credit and debit cards being cloned. But given that it's possible to perform online transactions using the details printed on the card, how does it add security?
Thats what the VbV and Mastercard SecureCode solutions are for (also known as 3D Secure)
With merchants that implement these solutions, a secondary password, or a OTP is required to complete the transaction. This secondary password\OTP is entered on the banks site and not on the merchants site.
This PDF describes the system (and its limitations)
EMV was designed to improve the security of card present transactions (i.e. physical purchases and ATM withdrawals) and to prevent magnetic stripe card skimming.
It's unfortunate that card not present transactions can still be performed using only information printed on the card.
As Akash has mentioned in his answer, there are solutions to that problem, but they are a long way from being universally implemented.