Google offers several API's for web and desktop programmers alike. They are often based around current Google products, including AdSense, Google Checkout and Google Toolbar. There are also Data APIs, which are aimed at higher end developers, who wish to manipulate Google's services on a more advanced level.


Google Web API

The Google Web API (or Google Web Services) is Google's public interface for registered developers. Using Simple Object Access Protocol (SOAP), a programmer can write services for search and data mining that rely on Google's results. Also, websurfers can view cached pages and make suggestions for better spelling.
Google Web API website

Google Web Toolkit

The Google Web Toolkit is an open-source toolkit by Google[6], as part of the Google Code initiative, to develop Ajax applications in the Java programming language. GWT supports rapid client/server development and debugging in any Java IDE. In a subsequent deployment step, the GWT compiler translates a working Java application into equivalent JavaScript that programmatically manipulates a web brower's HTML DOM using DHTML techniques. GWT emphasizes reusable, efficient solutions to recurring Ajax challenges, namely asynchronous remote procedure calls, history management, bookmarking, and cross-browser portability.

As of December 2006, the Google Web Toolkit contains widgets for:
Panels (AbsolutePanel, CellPanel, DeckPanel, DockPanel, and many more)
Buttons
Checkboxes
Radio Buttons
Tables (FlexTable, Grid, etc.)
Dialog Boxes
HTML primitives (Images, Hyperlinks, and raw HTML)
Menus and Menubars
Scrollpanes
Tabs
Trees

Google API stands for ‘Application Programmable Interface’. It can be used for a variety of purposes. For example to:
mine the data from complex industrial interfaces which are non-html,
research and analyze the discrepancies in certain data types used in various industries,
for getting regular and updated information on a given topic etc.