Cache: definition
A cache is a temporary copy of an already-computed result, kept so it doesn't have to be recalculated on every visit.
Why caching exists and where it lives
Generating an e-commerce page often requires several database queries and template compilation. Repeating that work for every visitor costs server time and slows the site down. The cache stores the result the first time and serves it again on later visits, as long as the content hasn't changed.
In PrestaShop 1.7 and later, the application cache sits in var/cache/prod (or var/cache/dev); in PrestaShop 1.6, compiled Smarty templates live in cache/smarty/compile and cache/smarty/cache. In WordPress, the cache depends on the plugin in use, and often sits alongside a server or CDN cache.
The cache is also the leading cause of confusion when troubleshooting: a price or text change that doesn't show up online is often explained by an old version still being served from the cache, rather than by a genuine update failure.
Frequently asked questions
Is there a risk to clearing the cache?
Why does the site stay slow even after clearing the cache?
Should the cache be disabled permanently to avoid this kind of issue?
Describe your need in one minute
A few targeted questions so I can reply with an estimate rather than another questionnaire.