This is a lightly edited version of a reply I wrote when a forum post asked some foundational questions about web development. Specifics they asked included "what's a server-side language for?", "is it the norm to use some kind of server like Apache?", and a variety of other things that prompted me to answer from quite basic principles. I'm including it here as something I can point at in the future :-)
The browser needs to get this HTML/CSS/JS from somewhere. Whilst this could simply be "local files on the disk of my computer", that's pretty useless if you ever want to show the site to someone not sitting at your keyboard. Instead, along with some other technologies, we use HTTP and DNS.
Hypertext Transfer Protocol (HTTP) is the language used to distribute a website's HTML/CSS/JS. A program which talks this language, an HTTP server such as Apache or Nginx, listens on a network socket for HTTP requests from browsers. The machine that the HTTP server process runs on is often called a web server, despite the confusion this sometimes brings!
Some of these requests will be for static assets - that is, the content being requested can be provided directly from a file on disk without any processing. The answer to a request is called a "response". Examples of responses are images (photographs as JPEGs, animations as GIFs, etc), HTML, CSS and JS, and sometimes data. Data is often formatted as JSON, as it's easy to work with in JS.
When the response is sufficiently complex or customised that it can't be served identically to everyone who requests it, the HTTP server might be configured not to serve the response directly from a file, but instead it may know about another process that it should pass the request on to for a customised response.
These processes are, very broadly, also servers, and are often also called web servers. They run code written in languages such as PHP, Python, Ruby, Go, JS, Java and C# to build the response and pass it back to the browser.
Different people use different languages, but generally don't mix and match without good reason, due to software maintenance costs and their own (mental) context switching costs. As always, different languages have their advocates, who prioritise different factors such as elegance, performance, safety, and library support, but generally it's a personal choice. Large projects or companies often have standard languages they ask their developers to use.
The Domain Name System
(DNS) is the glue which browsers use to translate a website's name
my-awesome-website.com) into the type of numeric network address which
computers can use to communicate. By buying that domain, you've rented the
right (from the people who own
.com) to tell people who ask about
my-awesome-website.com or any subdomain like
network address you like. In other words, you can point people who ask their
browser to show them
www.my-awesome-website.com towards the web server which
knows how to serve your website: the one we've configured above to respond with
static assets and possibly customised ("dynamic") responses.