* A local server, to cache and serve application resources (HTML, JavaScript, images, etc.) without needing to contact a server
* A database, to store and access data from within the browser
* A worker thread pool, to make web applications more responsive by performing expensive operations in the background
Google Gears is currently an early-access developers' release. It is not yet intended for use by real users in production applications at this time.
The plugin is a 700K download for Firefox 1.5+ and Internet Explorer 6.0+ that installs three developer APIs. One API will handle the creation of data objects to
store application information locally, another will be a SQLite relational database for searching the data, and the final part will enable asynchronous JavaScript so applications can sync data in the background without overburdening the browserAJAX applications are at the core of web development, providing lot of new opportunities. Here's a Presented by Aaron Boodman on Building Better AJAX applications using tools like Google Gears
Microsoft's domination over office suite software will be challenged. Google Apps (currently consisting of word processing, spreadsheets, email and calendar) has been out for awhile but hasn't gone anywhere in terms of widespread adoption. The biggest reason: you can't work when you're off-line. With Gears, this changes. Now, Google Docs and Spreadsheets still won't measure up to Word and Excel, but the appeal of being able to access and work on documents from anywhere is a big plus, and Gmail and Gcal are considered by many to be better at what they do than the Microsoft equivalents. It'll be awhile before there's real competition in this field, but Google took a major step toward being a competitor with the release of Gears.
More info on Google Gears