Yahoo! Developer Network helps software developers integrate their Web sites and applications with Yahoo! using standard technologies such as XML and RSS.
Yahoo! Web Services are a way for application developers to access content and services to build new applications. For example, you can combine Yahoo's data and services with those you create in a desktop application or those offered by other Web sites. With Yahoo! Web Services, more of the power of the Yahoo! network is in your hand (source: Yahoo Web Service FAQ)
In this series of lectures, we will give discuss how to access Yahoo! Web Services through Java. Most of the services are just as simple as sending URL requests and process the responses (in XML format). Most of the time we don't need extra software. Although you may like to apply an application ID (see an FAQ regarding the application ID) for your application. There is a limitation on how many request you can made but the limitation is on your IP address, not on the application ID.