What is the differences in the content of front end language and back and lauguage and their properties?

The front-end involves everything that the end-user sees. For example, you are a user of this forums, most of what you are seeing and interacting with on these forums as an end-user is a product of front-end development. Languages such as HTML, CSS and JavaScript directly effect how the end-user interprets and interacts with what they're seeing from their web browser.

The back-end involves manipulating & storing data, essentially the magic happening behind the scenes that you don't see much of (if any) as an end-user. Back-end development can be thought of as the "brains" of the application and exists primarily on servers and databases. Going back to the context of these forums, the files that make up the forums are stored on a web server somewhere (quite possibly several web servers). These files communicate with a database that stores all posts, users & a bunch of other data. PHP, Python and Ruby are a few examples of languages that deal with the backend. If I had to guess, I'd say that this forums is put together using PHP.

So to piece together the whole example, front-end languages are controlling how you view & interact with the forums in your web browser (client-side: HTML, CSS, JavaScript). Back-end languages are controlling how the data is manipulated and stored on the server, such as when you make a new question or post a reply (server-side: backend language such as PHP manipulating the data and storing said data in a database, such as a MySQL database).