The internet is no longer just a source of information, but it is a trading universe where thousands of credit and debit card transactions are carried out every second; private data is transmitted and stored online through systems which get breached at vulnerable points continously by the unscrupulous. Defenceless web applications are easy targets for hackers, malware, spambots and the like to breach your site and wreak their havoc.
The internet criminal can lurk anywhere, even in your own office. And he may be not just after money; the harvesting of users' private information can be put to use for all kinds of criminal intent on the black market - from the fairly innocuous but hugely annoying spammers, to serious fraud and theft of assets and identities. But you also find the hacker who does it for sport / spite.
The website builder needs to keep this awareness in uppermind when developing applications. Hackers are notorious for being geniuses of their craft, but do what you can to build your site DEFENSIVELY. You can find information anywhere on the internet on site security, but here are a few articles: