Here is a nice little video from F-Secure  which talks about how targeted attack works.  We all familiar with random attacks like a massive spam of a trojan spoofed email, but targeted attack is much stealthier — not many people could easily detect a targeted attack.  In a targeted attack, the responsible person for such attack is not going to hesitate in spoofing the attack to look like a real in-house email, document, and so on.  To spoof an attack with such details, the attacker has to go in length in gathering information about you, your organization, and the people that are working with you.  This is why a targeted attack could not be easily detected and most dangerous.