Three years ago, we added formal uptime monitoring to our review process, and the results show that most web hosts do an excellent job of keeping their sites up and running. If they don't, they suffer for it in our rankings. Even if they get everything else right, sites with uptime problems aren't eligible for top scores. All services suffer ups and downs, sometimes for reasons beyond their control. Those sites that fail to address the problem are penalized accordingly.
Cloud hosting is an alternative to hosting websites on single servers (either dedicated or shared servers) and can be considered as an extension of the concept of clustered hosting where websites are hosted on multiple servers. With cloud hosting, however, the network of servers that are used is vast and often pulled from different data centres in different locations.