In The Cloud
Every aspect of the system is stored "In The Cloud" but what does that actually mean?
Basically when you host something in the cloud, what normally happens is that the company supplying the cloud services (such as Amazon AWS, Windows Azure or Rack Space for example) manages everything for you such as storage, network resilience, load balancing and operating system updates.
That means that your data is Geo redundant (replicated across multiple data centres) so in case of a major failure at the primary data centre, all of your data is still available at the other data centre hundreds of miles away.
The databases are also replicated multiple times so that if the database server has a problem, you are automatically switched over to a "hot backup" to minimize down time.