jQuery
In plain terms
A classic library that simplified DOM manipulation and AJAX.
Going deeper
jQuery made cross-browser DOM access, events, and AJAX easy with concise syntax, dominating the web for years. Modern browsers and frameworks have reduced its need.
Under the hood
Smoothed over browser inconsistencies of its era; largely unnecessary now thanks to standardized DOM APIs.
Examples
Legacy websites, quick DOM scripts, WordPress themes, older plugins.
Think of it like...
Like a universal adapter that once made every browser speak the same language.