The rise of REST
When talking about Web APIs, it is imperative to mention the REST framework. Over the years, REST has proven to be a much simplified and efficient web service architecture style as compared to other architectures like RPC-based SOAP. The key reason for this popularity is because of its modern web design patterns and utilization of HTTP transport layer features to the fullest. REST has enabled many modern-world scenarios such as Mobile apps and Internet of Things (IoT), which would be challenging with protocols like SOAP because of its rigid schema-based WSDL structure and bulky XML standards.
Note
A good comparison of REST and SOAP protocols can be found at http://blog.smartbear.com/apis/understanding-soap-and-rest-basics/.