Both tools RPA and Selenium are completely different and they serve for different purposes. Talking about Robotic Process Automation(RPA), it is the technology that allows anyone today to configure computer software, or a “robot” to emulate and integrate the actions of a human interacting within digital systems to execute a business process. RPA robots utilize the user interface to capture data and manipulate applications just like humans do. Users can interpret, trigger responses and communicate with other systems in order to perform on a vast variety of repetitive tasks. Only substantially better: an RPA software robot never sleeps, makes zero mistakes and costs a lot less than an employee.
RPA software is costly and only MNC companies afford it.
On the other hand, Selenium is an automation testing tool used to test Web Applications which is totally opposite to RPA. Selenium is free and cannot be replaced because of its several features. I will suggest you take up the following Selenium training Course to learn it in a much better way. If you like reading the stuff then you must read the Selenium Tutorial. I am also mentioning a Selenium video tutorial to learn more about it..