Before diving into the concept it is good to know few sites we will be using for testing or practicing our selenium codes.
With below mentioned sites we can able to perform all our concept and scenario, giving us the grasps on the challenges when writing automation code.
The site are
1. https://phptravels.com/demo/ 2. http://thedemosite.co.uk/login.php 3. http://newtours.demoaut.com/ 4. http://the-internet.herokuapp.com/ 5. http://automationpractice.com/index.php 6. http://book.theautomatedtester.co.uk/ 7. http://www.seleniumframework.com/Practiceform/ 8. http://www.practiceselenium.com/ 9. https://www.saucedemo.com/ 10. https://jqueryui.com/droppable/
Most of the above sites are developed exclusively for automation testing which looks exactly like a real site that your company work on or you find out there in the internet.