Caching
If the user returns to a page fairly soon, it is likely that the data will not be retrieved from the source web server, as above, again. By default, browsers cache all web resources on the local hard drive. An HTTP request will be sent by the browser that asks for the data only if it has been updated since the last download. If it has not, the cached version will be reused in the rendering step.
This is particularly valuable in reducing the amount of web traffic on the internet. The decision about expiry is made independently for each resource (image, stylesheet, JavaScript file etc, as well as for the HTML itself). Thus even on sites with highly dynamic content, many of the basic resources are only supplied once per session or less. It is worth any web site designer collecting all the CSS and JavaScript into a few site-wide files so that they can be downloaded into users' caches and reduce page download times and demands on the server.
There are other components of the internet that can cache web content. The most common in practice are often built into corporate and academic firewalls where they cache web resources requested by one user for the benefit of all.
Apart from the facilities built into web servers that can ascertain when physical files have been updated, it is possible for designers of dynamically generated web pages to control the HTTP headers sent back to requesting users, so that pages are not cached when they should not be - for example internet banking and news pages.
This helps with understanding the difference between the HTTP 'GET' and 'POST' verbs—data requested with a GET may be cached, if other conditions are met, whereas data obtained after POSTing information to the server usually will not.

Web standards
At its core, the Web is made up of three standards:

the Uniform Resource Identifier (URI), which is a universal system for referencing resources on the Web, such as Web pages;
the HyperText Transfer Protocol (HTTP), which specifies how the browser and server communicate with each other; and
the HyperText Markup Language (HTML), used to define the structure and content of hypertext documents.
Berners-Lee now heads the World Wide Web Consortium (W3C), which develops and maintains these and other standards that enable computers on the Web to effectively store and communicate different forms of information.

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