The stack is the set of layers a request passes through. At the top is the front end: HTML for structure, CSS for the look and JavaScript for behaviour, which MDN names as the three core technologies and which every browser understands without installation. At the bottom is the back end: a program on a server, written in Python, JavaScript, Java, Go, PHP or another language, that receives requests over HTTP, applies the rules of the business, reads and writes a database, and answers. Between them runs the seam, usually a REST API exchanging JSON, and around them sit the components that job descriptions rarely list: version control, a build, a test suite, a deployment and a domain name.
| Layer | What it does | Typical choices |
|---|---|---|
| Front end | Renders the page and reacts to the user in the browser | HTML, CSS, JavaScript; a framework such as React, Vue or Angular |
| API | Defines how the two halves talk | REST over HTTP with JSON; GraphQL for many clients |
| Back end | Applies the rules and talks to the data | Node.js with Express, Python with Django, Java with Spring, PHP with Laravel |
| Data | Stores and queries records | PostgreSQL or MySQL; a document store where the data has no fixed shape |
| Delivery | Gets the code to users | Git, a pipeline, a host, DNS and TLS |
What the title means in practice
A full-stack developer can take one feature from the database to the screen without handing it to anyone. Depth on both sides is not required; the path of a request through every layer is.
The two halves demand different habits. Front-end work is about the user: layout that holds on a phone, state that updates without a reload, accessibility, and the performance budget of a page that has to load on a weak connection. Back-end work is about correctness and load: a request that must not double-charge a card, a query that must stay fast at a million rows, authentication that must not leak. Most developers settle on one side within two years and keep enough of the other to read a pull request and fix a small thing. The learning path runs in the same order as the curriculum MDN and freeCodeCamp both follow: pages first, then JavaScript, then a front-end framework, then a server and a database, then the API between them.
What the title is worth depends on the team. In a start-up of three engineers everyone is full stack by necessity, and the person who can deliver a feature end to end determines the team's throughput. In a company with separate front-end and platform teams the title rarely appears, and a developer is hired for one side, with the other treated as an additional qualification. The web development and back-end and API courses cover each side; the freeCodeCamp ladder from Responsive Web Design through the JavaScript and library certifications to Back End Development and APIs is a free, project-graded route across the whole of it.