A back-end developer creates the part of a website or application that lives on the server side of the internet. Using coding languages like PHP, JAVA, and Python, they're mostly tasked with creating the advanced functionalities of a product.
Back-end developers are a bit like plumbers. There aren't any public holidays in their honor, and we hardly ever think about their work, but boy, we could not live without them. They work on what is unseen, though used by everyone — on a daily basis.
A back-end developers works on the 'server side' of the internet. While you can see this website you're reading, the server this domain lives on maybe very far away, indeed. India, the US, or maybe Iceland? Using programming languages such as PHP, Java, and Python, back-end developers develop the functionalities of websites and applications on the 'back-end' of the internet. To do this, they need extensive knowledge of a wide range of web technologies and frameworks, such as Apacha, Django, and Laravel.
A back-end developer is responsible for more than just ensuring that the applications and websites you use function properly. They als take care of cybersecurity, which seems to become more complex (and important!) by the month. As more of our lives take place online, the extensive knowlegde and skills of back-end developers become increasingly indispensable.
The importance of online security adds a layer of complexity to the work of developers who work on the server-side. Their work isn't anything like what you see in movies, with futuristic landscapes made from flashing zeroes and ones zipping past — it's made out of text editors, with a lot of <>'s and ()'s.
Though I guess the difference is made clear by the names alone, a quick word on front vs. back-end development. A front-end developer is the professional concerned with making technology understandable and useable to people, while a back-end developer focuses on making people understandable to technology. They translate our language and tasks into queries and commands so that computers know what to do with us.