FAQ

/var/cache: Magento cache (config, blocks, layout etc). All cacheable objects except the page cache. This directory is empty if you use a third-party cache storage such as Redis.
/var/page_cache: Magento full page cache. Cached pages from the full p

/generated: It stores auto generated magento code (Proxy, Factories, Interceptors, etc...)
serialized map of all constructor definitions mixed with object linking configuration defined in di.xml
/var/view_preprocessed: It stores static contents b

 Flushing the cache clears the entire cache storage. If multiple Magento instances use the same cache storage, this action will affect such processes or applications. Use this action only once you've tried cleaning cache and you are still having t

_forward() : this internally forwards to another controller without changing the URL. Launch the below url. 
_redirect() : this represents a 301 or 302 redirect. It is used when a user needs to be redirected to a different URL.

The EAV database structure is employed because it is substantially more scalable than a normalised database structure. Without changing the basic database structure, developers can add properties to any entity (product, category, customer, order, etc.)