There's a continental divide in the Web Services world: the REST versus SOAP debate. A REST service has a low barrier to entry, and it makes possible the quick, grassroots adoption of a service. SOAP, on the other hand, is much more complex and requires more time and care to develop. It's favored by more traditional programmers and corporations because it has a formal structure.
Amazon was well aware of the REST versus SOAP debate when they designed their system. The choice between the two is not an easy one, as both offer certain advantages. Amazon wanted to attract everyone from the tinkering programmers to the corporate developers, so they implemented both SOAP and REST.
The following sections provide a quick look at each API interface.