Web Browser
A web browser is the application that retrieves web resources over HTTP and HTTPS, interprets them, and presents them as interactive pages, running the page's JavaScript and enforcing security rules along the way. Chrome, Safari, Firefox and Edge are the familiar examples; to a user the browser is the window onto the Web, and to a developer it is an extraordinarily capable runtime for applications.
The first browser, called WorldWideWeb, was written by Tim Berners-Lee in 1990 on a NeXT computer, but the program that made the Web popular was Mosaic in 1993. Developed at the National Center for Supercomputing Applications by Marc Andreessen and Eric Bina, Mosaic displayed images inline with text, ran on common platforms and was easy to install. Andreessen then co-founded Netscape, whose Navigator launched in 1994 and became dominant, introducing JavaScript in 1995. Microsoft's Internet Explorer followed in 1995, igniting the "browser wars." Later entrants include Safari (2003), Firefox (descended from Netscape's code, 1.0 in 2004) and Google Chrome (2008).
Turning a URL into pixels is a pipeline, and each stage is a distinct piece of engineering. The browser resolves the domain through DNS, opens a connection, performs the TLS handshake and sends an HTTP request. As the HTML response streams in, the parser builds the Document Object Model (DOM), a tree of elements. CSS is parsed into a parallel structure of style rules. Combining them yields a render tree of visible elements with computed styles, layout calculates each box's size and position, painting turns boxes into drawing commands, and compositing assembles layers, often on the GPU, into the final image. Script and image loads that block these steps are why careless pages feel slow.
Inside a browser, the rendering engine and the JavaScript engine do most of the work, and only a few engine families exist. Blink powers Chrome, Edge and many others; WebKit powers Safari; Gecko powers Firefox. JavaScript engines such as V8 compile script to machine code on the fly, fast enough that substantial applications run in a tab. WebAssembly lets code compiled from languages like C++ and Rust run at near-native speed, and APIs such as WebGPU give pages access to graphics and compute hardware, so some machine-learning models now run in the browser itself, with data never leaving the device. The result is that "which engine" matters more to compatibility than "which brand."
Security shapes the browser's design because it constantly runs code from strangers. Modern browsers isolate sites into separate sandboxed processes, enforce the same-origin policy so one site cannot read another's data, check certificates, and keep cookies, storage and permissions per origin. Users meet that model as permission prompts for camera, location and notifications. Extensions add power and also risk, since they can see everything the pages do. Keeping the browser updated matters because it is one of the most attacked pieces of software on any computer.
Several mix-ups are common. A browser is not a search engine: Google and Bing are websites that a browser visits, and mistaking the address bar for a search box hides how URLs work. A browser is also not the Internet, only one client of it, since email programs, games and apps speak the same protocols. And the page a user sees is not the file the server sent: it is the product of the DOM after script ran, which is why scraping a dynamic site with a plain HTTP client often returns something very different from the rendered page, and why developers use the browser's developer tools to inspect network requests, DOM and performance.
Read a web browser as a sandboxed program that fetches, parses and paints the Web: a pipeline from URL to DOM to pixels, combined with a script engine and a security model that lets untrusted pages run safely on your machine.