That partially depends on what you want to automate. From the front page of SeleniumHQ (my highlighting):
If you want to
- create quick bug reproduction scripts
- create scripts to aid in automation-aided exploratory testing
Then you want to use Selenium IDE; a Firefox add-on that will do
simple record-and-playback of interactions with the browser.
Conversely,
If you want to
- create robust, browser-based regression automation
- scale and distribute scripts across many environments
Then you want to use Selenium WebDriver; a collection of language
specific bindings to drive a browser -- the way it is meant to be
driven.
RC is deprecated:
Selenium WebDriver is the successor of Selenium Remote Control which
has been officially deprecated.
Selenium Server is for more complex cases:
The Selenium Server (used by both
WebDriver and Remote Control) now also includes built-in grid
capabilities.
Personally, after using both IDE and WebDriver:
- IDE is a relatively simple Firefox extension in which you can probably write useful tests within minutes of installing it.
- IDE isn't perfect; the issues may have been fixed in the meantime, but when I last used it (2013) it would sometimes not record my session, or would use 100% CPU until I killed it.
- WebDriver gives you the strength and complexity of a programming language:
- You can very easily reuse boilerplate code to get the system into a known state.
- You can implement unsupported operations in an OK language (Java) rather than an often critiqued one (JavaScript).