There are many companies who use Selenium internally to verify that their Web applications actually work in the browsers they are targeting. Browsers are not created equally and oftentimes have nuanced quirks in render and execution semantics. Selenium can be used to test things such as form submission, AJAX interactions on a page, and that certain elements appear on pages as they should.
Apart from these things, Selenium can also be used just as a general browser automation tool. This is not really its intended purpose but is a nice side effect of it being a testing tool. We use this to power the backend for Mogotest, for instance. If you want to make your career in Selenium you must take up the following Selenium training Course. If you like to read the stuff then you must read the following Selenium Tutorial. I am also mentioning a video of Selenium which you should watch to learn more about Selenium. .