This is the third and last part of a series (first post, second post).
Simply Asking For The Key
Although in the real life it's unlikely that a thief asks you for the key of your home, in the digital world this is the most common and successful type of attack. The technical name is phishing and, according to Wikipedia,
[It] is the attempt to acquire sensitive information such as usernames, passwords, and credit card details (and sometimes, indirectly, money) by masquerading as a trustworthy entity in an electronic communication.
I'm pretty sure that you have received at least one email from a bank that asked you to check the movements of your credit card. In the email there was a link to a fake website very similar to the bank's one. By logging in you simply give your password to a scammer.
The thing that makes possible this kind of attacks is called social engineering and it does its best in the internet era, even if it has been used probably since the dawn of time. It consist in a series of techniques that aims to make the victim perform actions that normally he wouldn't have done.
Sending tons of fake emails from the bank (or from Facebook, etc.) is similar to trawling. But there is also a technique called spear phishing which targets a specific person. To do this, the attacker starts to collect as many information as he can about the victim, included his mother's maiden name and the name of his first pet ;-)
Unfortunately, sometimes attacks are a little more direct.
Image from xkcd licensed under a Creative Commons Attribution-NonCommercial 2.5 License.