what is the difference between webservice and wcf

Web Service A Web Service is programmable application logic accessible via standard Web protocols.
One of these Web protocols is the Simple Object Access Protocol (SOAP).
SOAP is a W3C submitted note (as of May2000) that uses standards based technologies (XML for data description and HTTP for transport) to encode and transmit application data.
Consumers of a Web Service do not need to know anything about the platform, object model, or programming language used to implement the service; they only need to understand how to send and receive SOAP messages (HTTP and XML).
WCF Service Windows Communication Foundation (WCF) is a framework for building service-oriented applications.
Using WCF, you can send data as asynchronous messages from one service endpoint to another.
A service endpoint can be part of a continuously available service hosted by IIS, or it can be a service hosted in an application.
An endpoint can be a client of a service that requests data from a service endpoint.
The messages can be as simple as a single character or word sent as XML, or as complex as a stream of binary data.

Webservice can be considered as part of WCF, although it preexists WCF but in general WCF refers to all the technologies and methodologies that can be used in .NET framework, packed into a single platform, to communicate between two applications, be it on one system and OS or distributed across a heterogenous infrastructure.
WCF gives you the freedom of designing your communication and think about the protocol later! it can be through web services or MSMQ or any other forms of inter-process communication available.
the beauty of WCF is that you don't have to be worried about the actual implementation! and you certainly have the ability to change the protocol if the requirements of your project may change or simply you change your mind.