SOAP uses a standard format SOAP envilop in every call, and described using XML based languague called WSDL.

HTTP service is light weight as doesnt require any envilop in interactions. SOAP can be used on HTTP,FTP and SMTP transport 

protocols where as HTTP service has limited to only HTTP protocol.SOAP always do POST operaton where as HTTP can configure to use GET, POST,PUT or DELETE operations.

Ref:http://blog.smartbear.com/apis/understanding-soap-and-rest-basics/