Webservices/ WCF/ MVC

http://www.youtube.com/watch?v=cUFfE7qOU9Q&feature=bf_next&list=SPCD7D098B66C806CA&lf=list_related.

http://dotnetguts.blogspot.in/2007/09/all-about-web-service-in-net.html

http://www.codeproject.com/Articles/139787/What-s-the-Difference-between-WCF-and-Web-Services
http://wcftutorial.net/Difference-between-WCF-and-Webservice.aspx

Synchronous is one after another, so you send one email when that process is done it sends the next one. Asynchronous starts sending an email and it doesn't care if the first one is done and will start the second, and this goes on until all emails are sent. Normally, an asynchronous process would open a new thread and run in the background.