According to ProgrammableWeb, Apple Siri uses more than 35 APIs . For instance, Siri accounts for about a quarter of the queries fielded by Wolfram|Alpha . Yelp is another of the APIs Siri uses .
Originally, Siri was built on SRI International CALO (Cognitive Assistant that Learns and Organizes) . What is not so widely known is that SPARK (SRI Procedural Agent Realization Kit)  forms the heart of CALO’s task execution.
It’s fairly certain that Nuance Communications ASR (Automatic Speech Recognition) is behind Siri . And in terms of Cloud Computing, apparently Apple’s new solar-powered data center in North Carolina “is handling at least part of the workload for Apple’s Siri voice command system” .