This one is also referred to as the "Stranded Scam". This will come from someone you know - either a friend or at least an acquaintance that has your email address in their contact list.
What happens is that the scammer will get access to that person's email account, typically their yahoo, gmail, or hotmail.com. They do this by using software that goes through all kinds of permutations of password combinations and once in awhile they will hit it right. The other more common way is they used "phishing" technique and gotten that person to reveal their password.
Once the scammer gets into the account, he sends out an email to everyone in that person's contact list. So it will look like it is coming from someone you know because it IS coming from the account of someone you know.
The email will have some story about they are traveling and their passport and cash got stolen and asking you to wire them emergency money to get home. You would not believe how many people actually do this. Who is there to pick up the cash? The scammer, of course, who then disappears and is untraceable.
My advice - if you get one of these emails, just CALL your friend before wiring any money and if you can't get a hold of them... don't wire money. Decide you are going to speak to them verbally before taking any action.