As you can imagine, the related security issues are very complex, certainly too complex to cover in depth here. But essentially the main means of protecting data and systems from outside interference is the so-called firewall. If a retailer has a network of PC's, then imagine that network as a castle. The firewall is the network's equivalent of a guardhouse and drawbridge. People can only enter and leave a castle via the drawbridge.
Information can only enter and leave the retailer's PC network through the firewall.
Like the guards, the firewall makes decisions based on pre-defined criteria. A guard may refuse entry to anyone not carrying the right papers. The firewall may block access to the network if this access is being requested by a computer at a location not on an approved list. Although firewalls can provide robust protection against intrusion, they are not failsafe. For example, someone with the right password might be able to bypass the firewall, just as a spy could forge the right documents to get into a castle.