Which Selenium Tool is appropriate for testing?

It depends on the usage /purpose .
Selenium IDE is a Firefox add-on that will do simple record-and-playback of interactions with the browser.

1. create quick bug reproduction scripts
2. create scripts to aid in automation-aided exploratory testing

Selenium WebDriver is a collection of language specific bindings to drive a browser — the way it is meant to be driven. Selenium WebDriver is the successor of Selenium Remote Control (Selenium RC) which has been officially deprecated. The Selenium Server (used by both WebDriver and Remote Control) now also includes built-in grid capabilities.

 • create robust, browser-based regression automation
• scale and distribute scripts across many environments

Selenium Grid is used when you want to distribute your test across multiple devices/machines

 • Selenium Grid allows you to run your tests in parallel, that is, different tests can be run at the same time on different remote machines. This has two advantages.
           o First, if you have a large test suite, or a slow-running test suite, you can boost its performance substantially by using Selenium Grid to divide your test suite to run different tests at the same time using those different machines.
           o Also, if you must run your test suite on multiple environments you can have different remote machines supporting and running your tests in them at the same time.
• In each case Selenium Grid greatly improves the time it takes to run your suite by making use of parallel processing.