Selenium FIT mode vs. Driven mode

Selenium has two usage modes:
1)FIT mode: table style simple script which usually run on Selenium CORE/IDE in standalone IE/FF. We also call it as 'Selenese'. It's simple and easy to install/use for QA with no programming skills.
This is more for "black box" testing and is "asynchronous" in design.
You send the whole test script over to the browser in a "REST" fashion, it generally only has the same access to click and type and change things as a regular user would.

2)Driven mode: a language specific driver for Selenium. It used a proxy server to bypass Same Origin Policy for Selenium. Selenium RC is the product for this mode.
This enables "white box" testing and also "synchronous" in design.
With white box testing, you can send one command to the browser in an "RPC" fashion, get a result, then check the back-end database to confirm like Unit Test. You can benifit from programming lauguage like Java for Selenium.

Read Jason Huggins' topic: Choosing between "Driven Mode" and "FIT or Table style"