You can use Data Loader interactively through its user interface, or set up automated batch processes launched from the command line. When you use the user interface, you work interactively to specify the configuration parameters, CSV files used for import and export, and the field mappings that map the field names in your import file with the field names in Salesforce. When you set up batch processes through the command line, you specify the configuration, data sources, mappings, and actions in files used for automated processing.

The Data Loader offers the following key features:

• An easy-to-use wizard interface for interactive use
• An alternate command line interface for automated batch operations
• Support for large files with up to 5 million records
• Drag-and-drop field mapping
• Support for all objects, including custom objects
• Detailed success and error log files in CSV format
• A built-in CSV file viewer
• Support for Windows 7 and Windows XP

In addition, an open source version of Data Loader is available here.

The Data Loader complements the web-based import wizards that are accessible from the Setup menu in the online application. Refer to the following guidelines to determine which method of importing best suits your business needs:

Use the Data Loader when:

• You need to load 50,000 to 5,000,000 records. If you need to load more than 5,000,000 records, we recommend you work with a Salesforce.com partner.
• You need to load into an object that is not yet supported by web-based importing.
• You want to schedule regular data loads, such as nightly imports.
• You want to export your data for backup purposes.

• You are loading less than 50,000 records.
• The object you need to import is supported by the web-based import wizards.
• You want to prevent duplicates by uploading records according to account name and site, contact email address, or lead email address.