Modern software development processes want to develop software quickly and reliably. To reduce the risk of regression defects, they automate the application testing process to allow fast and reliable checking of test conditions after each change.
If you are working on web applications, then the first automation tool you will want to get familiar with, is Selenium.
When we speak of Selenium, we are actually talking about both its flavours; Selenium IDE and Selenium WebDriver. Both automate browsers in support of web application testing.