The third-party Snapchat client for Windows Phone, Swapchat has been removed from the WP Store due to a copyright infringement notice from Snapchat. The application was developed by SRC Apps by reverse engineering the Snapchat API and using them to build an app for Windows Phone. While there could be some good potential in this move by Snapchat (official client on the way), Windows Phone users will be left with no alternative until it happens.
If you’ve already got Swapchat installed on your phone then you can continue to use the app as normal. Providing that Snapchat don’t find a way to stop the app from operating. If you haven’t got the app installed, then you’ll just have to wait until an official app is released!