They do this by posing as a man or woman in another country or even overseas and attempt to be interested in you and provide their email address so that you can continue communicating offline.  In most cases scammers will use email to try and start an online relationship with you.  Once they feel that trust has been established they will make up a story about an emergency and then ask for money or personsal information.  You can avoid being a victim of these scams by noticing these warning signs:

Asking for Money
This is the most ovious sign of a scam. It's just not normal to ask for money if you've never met someone in person. Scammers are very creative when it comes to asking for money.   First, they may ask you to help with airfare so they can come and visit you, or help out with an emergency and so on. Never send money!

Initiating First Contact
Scammers will almost always be the first to initiate contact with you. If you get a message from someone overseas, it's a good idea not to respond.

Scammers Prefer Communicating with Email
A scammer will want to communicate with email as soon as possible because they know their profile could be deleted soon. It's common for a scammer to include their email address in their inital messages to you and ask you to email them.  

Fake Pictures
Scammers will often use photos from models that are attractive. If you feel that the picture is suspicious then chances are it probably is a fake. Another approach they will use involves stealing someone else's photo and using it. 

Messages Vary
When you're exchanging emails with someone, watch out for inconsistencies.  There are several things you can watch out for such as a variation in the style of the email, repetitive statements, etc.. The scammer who initiated the communication with you might not  always be the same one that follows it through. If something doesn't seem right about the emails you are receiving, then stop emailing them.

Besides looking out for red flags you can take other measures to further protect yourself:

1) Always initiate communication, it’s safer.

2) Guard your privacy.

3) Never send money.

4) Trust your instincts from the beginning.

5) Never reply to someone who immediately includes their email address for you to continue the communication.

It’s best to always be on your guard when using online dating services. Don't get scammed!