4-1-9 Schemes frequently use the following tactics:

An individual or company receives a letter or fax from an alleged "official" representing a foreign government or agency.

An offer is made to transfer millions of dollars in "over invoiced contract" funds into your personal bank account.

You are encouraged to travel overseas to complete the transaction.

You are requested to provide blank company letterhead forms, banking account information, telephone/fax numbers.

You receive numerous documents with official looking stamps, seals and logo testifying to the authenticity of the proposal.

Eventually you must provide up-front or advance fees for various taxes, attorney fees, transaction fees or bribes.

Other forms of 4-1-9 schemes include: c.o.d. of goods or services, real estate ventures, purchases of crude oil at reduced prices, beneficiary of a will, recipient of an award and paper currency conversion.
Nigerian Advance Fee Fraud Overview

The perpetrators of Advance Fee Fraud (AFF), known internationally as "4-1-9" fraud after the section of the Nigerian penal code which addresses fraud schemes, are often very creative and innovative.
Unfortunately, there is a perception that no one is prone to enter into such an obviously suspicious relationship. However, a large number of victims are enticed into believing they have been singled out from the masses to share in multi-million dollar windfall profits for doing absolutely nothing. It is also a misconception that the victim’s bank account is requested so the culprit can plunder it — this is not the primary reason for the account request — merely a signal they have hooked another victim.

In almost every case there is a sense of urgency.

The victim is enticed to travel to Nigeria or a border country.

There are many forged official looking documents.

Most of the correspondence is handled by fax or through the mail.

The confidential nature of the transaction is emphasized.

There are usually claims of strong ties to Nigerian officials.

A Nigerian residing in the U.S., London or other foreign venue may claim to be a clearing house bank for the Central Bank of Nigeria.

Offices in legitimate government buildings appear to have been used by impostors posing as the real occupants or officials.

The most common forms of these fraudulent business proposals fall into the following main categories:

Disbursement of money from wills
Contract fraud (C.O.D. of goods or services)
Purchase of real estate
Conversion of hard currency
Transfer of funds from over invoiced contracts
Sale of crude oil at below market prices

The most prevalent and successful cases of Advance Fee Fraud is the fund transfer scam. In this scheme, a company or individual will typically receive an unsolicited letter by mail from a Nigerian claiming to be a senior civil servant. In the letter, the Nigerian will inform the recipient that he is seeking a reputable foreign company or individual into whose account he can deposit funds ranging from $10-$60 million that the Nigerian government overpaid on some procurement contract.

The criminals obtain the names of potential victims from a variety of sources including trade journals, professional directories, newspapers, and commercial libraries. They do not target a single company, but rather send out mailings en masse. The sender declares that he is a senior civil servant in one of the Nigerian Ministries, usually the Nigerian National Petroleum Corporation (NNPC).

Victims are often convinced of the authenticity of Advance Fee Fraud schemes by the forged or false documents bearing apparently official Nigerian government letterhead, seals, as well as false letters of credit, payment schedules and bank drafts. The fraudster may establish the credibility of his contacts, and thereby his influence, by arranging a meeting between the victim and "government officials" in real or fake government offices.

In the next stage some alleged problem concerning the "inside man" will suddenly arise. An official will demand an up-front bribe or an unforeseen tax or fee to the Nigerian government will have to be paid before the money can be transferred. These can include licensing fees, registration fees, and various forms of taxes and attorney fees. Normally each fee paid is described as the very last fee required. Invariably, oversights and errors in the deal are discovered by the Nigerians, necessitating additional payments and allowing the scheme to be stretched out over many months.

Indications are that Advance Fee Fraud grosses hundreds of millions of dollars annually and the losses are continuing to escalate. In all likelihood, there are victims who do not report their losses to authorities due to either fear or embarrassment.

END

Sorry for the long post, but hope this clarifies the matter.
References :
http://www.419eater.com/html/419faq.htm

There is no lottery.

There is no Shell, BBC, Yahoo, Coca-Cola, MSN, Microsoft, BMW or any other company in the entire world that sponsors a lottery that notifies winners via email, phone call or text.

There is only a scammer trying to steal your hard-earned money.

The next email will be from another of the scammer’s fake names and free email addresses pretending to be the "lottery official" and will demand you pay for made-up fees and taxes, in cash, and only by Western Union or moneygram.

Western Union and moneygram do not verify anything on the form the sender fills out, not the name, not the street address, not the country, not even the gender of the receiver, it all means absolutely nothing. The clerk will not bother to check ID and will simply hand off your cash to whomever walks in the door with the MTCN# and question/answer. Neither company will tell the sender who picked up the cash, at what store location or even in what country your money walked out the door. Neither company has any kind of refund policy, money sent is money gone forever.

Now that you have responded to a scammer, you are on his ‘potential sucker’ list, he will try again to separate you from your cash. He will send you more emails from his other free email addresses using another of his fake names with all kinds of stories of great jobs, lottery winnings, millions in the bank and desperate, lonely, sexy singles. He will sell your email address to all his scamming buddies who will also send you dozens of fake emails all with the exact same goal, you sending them your cash via Western Union or moneygram.

Do you know how to check the header of a received email? If not, you could google for information. Being able to read the header to determine the geographic location an email originated from will help you weed out the most obvious scams and scammers. Then delete and block that scammer. Don’t bother to tell him that you know he is a scammer, it isn’t worth your effort. He has one job in life, convincing victims to send him their hard-earned cash.

Whenever suspicious or just plain curious, google everything, website addresses, names used, companies mentioned, phone numbers given, all email addresses, even sentences from the emails as you might be unpleasantly surprised at what you find already posted online. You can also post/ask here and every scam-warner-anti-fraud-busting site you can find before taking a chance and losing money to a scammer.

If you google "fake yahoo lottery", "lotto Western Union fraud" or something similar, you will find hundreds of posts of victims and near-victims of this type of scam.
References :
http://scam.com
http://scamwarners.com

Yahoo has a whole page warning about this scam using their name
http://help.yahoo.com/l/us/yahoo/mail/yahoomail/abuse/abuse-63.html
"If you get an email that looks like it’s from Yahoo! but tells you you’re the winner of a Yahoo! Lottery or other contest – and it asks you to email personal information to claim a cash prize or reward – click Spam to dispose of it. You can also report this suspicious email by going to our Abuse Form and file a complaint."
References :

if he wants to send Western Union then he does a basic Western Union transfer and sends you the MTCN number so you can pick up cash at any WU office.
References :

http://www.fraudwatchers.org/forums/showthread.php?t=20200
http://www.419scam.org/emails/2010-01/19/00997074.36.htm
http://www.419baiter.com/_scam_emails/08-09f/scam-email-37553.html
References :