Google Spreadsheets is the spreadsheet component of the Google Docs suite. This was Google's first web-based application and, as befits its longevity, is also Google's most sophisticated app.
The Google Spreadsheets workspace looks a lot like that of Microsoft Excel. You'll quickly recognize the familiar row-and-column grid, topped by a series pull-down menus and an easy-to-use formatting toolbar.
The latest version of Google Spreadsheets contains much functionality lacking in the original release. For example, Google Spreadsheets now contains a raft of formulas and functions, almost as many as available with Excel; it also lets you insert multiple pages within a single spreadsheet file and create all manner of pie, line, and bar charts.
While Google Spreadsheets is similar in features to Excel, it's not identical. In particular, Google Spreadsheets lacks some of Excel's more advanced features, such as macros, pivot tables, and databases. You do, however, gain the sharing and collaboration features inherent in any web-based application.
In all, Google Spreadsheets offers a mix of features and functionality that should appeal to beginning and casual spreadsheet users. More sophisticated users, however, will bemoan its lack of advanced features.