Backend development is centered in building the architecture that supports a web application, using a diverse range of languages and systems. Once you choose the language you like most, you will usually need to choose a web framework as well. And besides this, you have a wide range of databases, search engines and queue systems to choose from.
A web stack is like a stack of books. The upper item is the application the user can interact with, while the one at the bottom is the base: the operating system. All the items between them are the technology needed for both the application and the operating system to communicate to each other. All of them are a team with a common goal: provide everything the application may need to work.