Backend code runs on the server. It handles HTTP requests, talks to databases, calls other APIs, runs background jobs, and returns JSON (or other formats) to clients. The language you choose affects performance, ecosystem, hiring, and how you structure code.

There is no single "best" language; each has trade-offs. Pick one, learn it well, then add another. The concepts (APIs, databases, auth) transfer across languages.

Runtime: Interpreted; you run python app.py or use ASGI/WSGI servers (e.g. Gunicorn, uvicorn). No compilation step.

Typing: Dynamically typed by default; type hints (Python 3.5+) improve tooling and readability. Ecosystem: Huge for data, scripting, and web: pip, Django, FastAPI, Flask. Concurrency: GIL limits CPU parallelism; use asyncio for I/O-bound work or multiprocessing for CPU-bound. Typical use: APIs, scripts, ML pipelines, internal tools.

Runtime: Compiled to bytecode; runs on the JVM. You build a JAR or use a framework that runs an embedded server.

Typing: Statically typed. Strong tooling (IDEs, refactoring). Ecosystem: Maven or Gradle; Spring Boot dominates for web and microservices. Concurrency: Mature (threads, CompletableFuture, reactive). Typical use: Large enterprise backends, high-throughput services, Android.

Runtime: Compiled to a single binary; no runtime dependency. go build produces an executable.

Typing: Statically typed; simple type system. Ecosystem: go mod; standard library is strong; Gin, Echo for HTTP. Concurrency: Goroutines and channels are first-class; great for many concurrent connections. Typical use: APIs, CLIs, infra tooling (Docker, Kubernetes are written in Go).

Runtime: V8 (Chrome’s engine); JavaScript on the server. node app.js or ts-node for TypeScript.

Typing: JavaScript is dynamic; TypeScript adds static types and is widely used. Ecosystem: npm; Express, Fastify, NestJS for HTTP. Concurrency: Single-threaded event loop; non-blocking I/O for many connections. Typical use: APIs, real-time apps, full-stack teams using one language.

If you are new: Python or Node.js get you building APIs quickly. If you care about performance and concurrency: Go. If you are in an enterprise or Spring shop: Java.

Learn: variables, functions, error handling, and how to run and test the app. Then add: package manager, HTTP server, database driver, and environment config. The rest (auth, caching, queues) is similar across languages.