Electronic Cash is fairly new payment method  for e-commerce.  Read on wikipedia about Electronic money, which is also known as electronic cash

Early form of e-cash is introduced by David Chaum. David Chaum invented Digital cash. Read about it here.  (For additional reading, click on Electronic Cash)

PayPal is an example of Electronic cash System. Read about PayPal and  visit How Does it Work?

Merchants and developer should first test their PayPal transaction in  PayPal sandbox environment.   (Sandbox User guide)

Additional documentation from PayPal are available here