Snapchat is an instant messaging app that is focused on sending pictures.
You can share images you take for up to 10 seconds and then the image cannot be accessed again. To view the image you have to hold the message which starts the timer. Even if you stop touching the screen, the countdown timer will continue. Snapchat now supports video messages.
Snapchat uses this to keep images secure, but there is no way of knowing if the images are archived somewhere else. There is also the danger of inappropriate images being shared. When you start Snapchat, you use your phone number as an ID or you can connect to friends through Facebook and Twitter.
As a communication app, it does not offer the same abilities of traditional messaging apps as you cannot send text. Still it does give a means of sending disposable messages to friends.
Snapchat is a diversionary app that is just for fun.
Clicking the 'Buy Now' button will take you to a page in the App Store, where you can purchase or download the app.