To use Selenium, you need to download the Selenium Server (only a jar file), and the language-specific client drivers that corresponds to the language of your preference (Java, Python, Ruby, etc.). You find all these in the same place:
http://docs.seleniumhq.org/download/
Once you perform all the necessary steps for installation (downloading and installing libraries), on your code you will need to import the libraries that will allow you to interact with the selenium server, and perform your automation tasks.
For basic examples on what can be done and how (with actual code and snippets) you can refer to the official Selenium website:
Setting up a Selenium WebDriver Project. They have examples in many languages.
With this automation tool, plus some knowledge of general computer science, you can create many useful scripts, even for personal use. If you think 'automation', think also outside of Selenium by programming general software, and add the possibility of those scripts interacting with Selenium itself, or even, using virtual machines to interact and not disturbing your host machine. If you have enough computer power, you can even multiply these to get parallel computing possibilities.
Another useful thing that many do not realize is that, while using certain scripting languages you require the server to be started, that can also be automated very easily from within the code. You can create processes, threads, and make calls to the OS to run Selenium Server and close it at will.
For people working professionally on testing, they probably already know plenty about automation. Selenium adds possibilities to integrate it's findings, and by enabling us the ability to create behavior and logic with our regular programming abilities, we can create reports on many formats and tools enabling further interaction with other software being used by any company working on software development and testing.
