Who? The person who develops the core of web sites. On contrary to Front End developers, who work on User Interface (what we normally see), the work of backend developer pushes gears under the hood. However, the job is very vesatile and provides many opportunities for creative thinking and innovating.
How to become? Most programming languages suitable for web developement, have mainstream web frameworks. Pick one you like and start learning the programming language simulteniously developing simple web-site. Basic understanding of internet principles is encouraged. Next proceed to data processing (SQL, files) - it will allow you to build simple interactive websites. General understanding of authorization/authentication algorithms would be profitable. Once you will confident with your basic skills or unsatisfied with performance of your site, you could move to more complex topics like caching, optimizations, balance loading etc.
Track your progress by checking the skills below. You will see what other alternatives are open to you and where to read/watch about given skill. Switch to graphical view (roadmap) if you prefer picutures to texts
Web Application Security Threats
Cross-origin resource sharing (CORS)
Same-origin security policy
Teamwork
Book
Leaders Eat Last: Why Some Teams Pull Together and Others Don't: Sinek, Simon: 9781591848011: Amazon.com: BooksBook
The Catalyst Effect: 12 Skills and Behaviors to Boost Your Impact and Elevate Team Performance: Toomer, Jerry: 9781787435520: Amazon.com: BooksBook
The 5 Languages of Appreciation in the Workplace: Empowering Organizations by Encouraging People: Chapman, Gary, White, Paul: 9780802418401: Amazon.com: Books
Intellectual property
Book
Leaders Eat Last: Why Some Teams Pull Together and Others Don't: Sinek, Simon: 9781591848011: Amazon.com: BooksBook
The Catalyst Effect: 12 Skills and Behaviors to Boost Your Impact and Elevate Team Performance: Toomer, Jerry: 9781787435520: Amazon.com: BooksBook
The 5 Languages of Appreciation in the Workplace: Empowering Organizations by Encouraging People: Chapman, Gary, White, Paul: 9780802418401: Amazon.com: Books
Cloud technologies
Performance Optimization
High Availability Principles
Continuous Integration
Continuous Delivery
Software Development Management Frameworks
Software Testing Techniques
Containers
Version Control Systems
Linux Developer
Book
Linux for Developers: Jumpstart Your Linux Programming Skills [Book]Article
What should developers know about UNIX-based systems? - Software Engineering Stack Exchange
Windows Developer
Book
Linux for Developers: Jumpstart Your Linux Programming Skills [Book]Article
What should developers know about UNIX-based systems? - Software Engineering Stack Exchange
Operating Systems
Book
Linux for Developers: Jumpstart Your Linux Programming Skills [Book]Article
What should developers know about UNIX-based systems? - Software Engineering Stack Exchange
Web Servers
Book
Linux for Developers: Jumpstart Your Linux Programming Skills [Book]Article
What should developers know about UNIX-based systems? - Software Engineering Stack Exchange
Data transfer
Data retrieval
Data Formats
User Interface Disign Patterns
Multithreading
Program architecture
Backend Programming Language
IP Network
DNS
IP Address
To see a interactive roadmap click on picture