To get started, download the API and register yourself to receive a key. Each individual using the Google API must have his or her own key; each key is limited to 1,000 calls per day, and each call can return a maximum of 10 search results.
The Google Web API is a web service exposed by a Web Service Definition Language (WDSL) file. This article isn't a tutorial on web services, and in fact you don't need to know much about web services to use the API, but a quick overview wouldn't hurt.
A web service is an application, process, or function whose interface is exposed to the web, enabling other services to utilize the web service's functionality regardless of where they are or in what language(s) they were written. A WSDL file is an XML document that defines where the web service is located, what it does, and what kind of data it uses. The data traveling to and from web services—for example, a search request or the results of a search—is passed using the Simple Object Access Protocol (SOAP), which handles the details of the message encapsulation and delivery.
If you're a .NET programmer, Microsoft has made it incredibly easy for your applications to work with web services. On the Java side, although you can use Google's WSDL file and an application server to access their services, the Google developers thoughtfully included a standalone Java library that handles all that stuff, removing the need for anything other than the Java SDK.
Regardless of whether you're using .NET or Java, you have access to the same functionality, so we'll start with a simple example using C#, and then delve a little deeper in Java.