A back-end developer builds and maintains the technology that powers those components which, together, enable the user-facing side of the website to even exist in the first place.
Java is a class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible.
C++ is a general-purpose programming language created by Bjarne Stroustrup as an extension of the C programming language, or "C with Classes"
Ruby on Rails, or Rails, is a server-side web application framework written in Ruby under the MIT License. Rails is a model–view–controller framework, providing default structures for a database, a web service, and web pages.
Django is a Python-based free and open-source web framework that follows the model-template-views architectural pattern. It is maintained by the Django Software Foundation, an American independent organization established as a 501 non-profit.
ASP.NET is an open-source, server-side web-application framework designed for web development to produce dynamic web pages. It was developed by Microsoft to allow programmers to build dynamic web sites, applications and services.
Flask is a micro web framework written in Python. It is classified as a microframework because it does not require particular tools or libraries. It has no database abstraction layer, form validation, or any other components where pre-existing third-party libraries provide common functions.