WSDL 1.0 (Sept. 2000) was developed by IBM, Microsoft, and Ariba to describe Web Services for their SOAP toolkit. It was built by combining two service description languages: NASSL (Network Application Service Specification Language) from IBM and SDL (Service Description Language) from Microsoft.
WSDL 1.1, published in March 2001, is the formalization of WSDL 1.0. No major changes were introduced between 1.0 and 1.1.
WSDL 1.2 (June 2003) was a working draft at W3C, but has become WSDL 2.0. According to W3C: WSDL 1.2 is easier and more flexible for developers than the previous version. WSDL 1.2 attempts to remove non-interoperable features and also defines the HTTP 1.1 binding better. WSDL 1.2 was not supported by most SOAP servers/vendors.
WSDL 2.0 became a W3C recommendation on June 2007. WSDL 1.2 was renamed to WSDL 2.0 because it has substantial differences from WSDL 1.1. The changes are the following:
Added further semantics to the description language
Removed message constructs
Operator overloading not supported
PortTypes renamed to interfaces
Ports renamed to endpoints