SSL gives you the ability to communicate securely with your web customers. When using SSL (Secure Socket Layer) all communications with your clients are protected so customer card details are captured, encrypted and held on a secure server. You, as site owner, can then collect the card details and processes the payment personally.
When using a Third Party Payment Processor the 'checkout' process is handled by them. All credit card details are captured by the payment processor, encrypted and securely handled using SSL. As the merchant, you do not have any involvement with credit card details. The payment processor will pass on the payment to you along with the order which you must then fulfill.
