Obviously follow Paul's advice for greater understanding but my take on it would be:
CORBA has the key advantage of being truly object-oriented component architecture and comes with a host of standard services like naming, transactions, printing, events etc which support complex applications in a heterogeneous environment in a platform and language independent manner. For example, there is no equivalent of CORBA Security service in web service environment and only now the suppliers have started addressing these aspects. So the disadvantage of web services over CORBA is that it is reinventing-the-wheel and cannot handle the required amount of complexity in its current state. However the complexity of CORBA, the issues with its specification and the slowness of new development have opened way for web services which are more suited to web-environment with their loose coupling, XML and HTTP usage. The SOAP messages of web services are simple to use and implement. Unsurprisingly, this simplicity has given momentum to web services over CORBA.