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Clients are computers that access the web in any way.
What's different across these devices?
Browsers are programs that render content on the web, most often HTML pages and related resources.
HTML is a "markup language" that describes a webpage. When you're on a webpage, you can "view source" to see the HTML that the browser is rendering.
Various companies and organizations provide browsers, and each browser varies in how it displays webpages.
Browser popularity varies year by year. Developers must decide which browsers they want their webpage to work well in.
Servers are just programs running on a computer that serve up content.
Sometimes they serve up static files, and are exposed like a file system.
Sometimes they can be scripted using "server-side" programming languages (PHP, Python, Perl, Java, etc.) to serve more dynamic pages.