What is the difference between Selenium IDE, selenium Webdriver and selenium Grid?

WebDriver is an API for programming tests in a variety of languages in Selenium 2, which can run with or without a server.

Selenium IDE is an integrated development environment for Selenium tests. It is implemented as a Firefox extension, and allows you to record, edit, and debug tests. Selenium IDE includes the entire Selenium Core, allowing you to easily and quickly record and play back tests in the actual environment that they will run.

WebDriver is an API that drive a browser natively as a user would either locally or on a remote machine using the Selenium Server it marks a leap forward in terms of browser automation.

Selenium-Grid allows you run your tests on different machines against different browsers in parallel. That is, running multiple tests at the same time against different machines running different browsers and operating systems. Essentially, Selenium-Grid support distributed test execution. It allows for running your tests in a distributed test execution environment.