Azure Wordpress and Scalable Wordpress
Basic differences: Media is stored in a separate storage account and MySQL database is automatically scalable using ClearDB (master-master replica).
Regular Wordpress is single site single database only. For performance critical scenarios, scalable wordpress will handle a lot more concurrent requests. It also provides contingency.
Scalable WordPress on Azure (using cleardb) walks you through the steps to turn on auto-scaling technologies - using some expensive Azure VMs. You can find more information here: http://bit.ly/1Mi7bYE
I had a poor experience with Cleardb - their support was non-existent. I"ve had much better luck with Bitnami's implementation of WordPress. However, you'll have to do some extra work to edit out their banners that "magically" appear on your WordPress site.