Backend
The half of an application users never see: server code that stores data, enforces the rules, and answers requests from the screen.
What it tells you about a developer
Strong backend developers think about correctness, failure, and scale: what happens when a payment retries, or when traffic triples. The skill transfers well across companies and across languages, because the hard part is systems thinking. Backend depth usually anchors a senior generalist, and it is the most common dominant side under a full-stack title.
Backend code runs on servers the company controls. It handles the parts where mistakes are expensive: sign-in, payments, data storage, and serving many users at once. Common backend languages are Python, Go, Java, and TypeScript, paired with a framework and a database such as Postgres.