Web Hosting is at it's simplest definition the storage and display
of content for a website. You open a web hosting account, you
upload your content, and the web host accepts request to view your
content by showing the requested content. Pretty simple.
Doing it right though can be another matter. Problems arise at
the beginning with the creation of the website. So many websites
have "missing" pages/links (404's in webspeak). The web host only
responds with the requested content. If the requested content
does not exist... the user is served an error page (unless you use a
custom 404 page).
Even if you have all of your ducks in a row (all links are correct
and content exists), once you upload the content your web host takes
control of the process. There can be downtime on the server
(there will be downtime). The downtime can be due to several
factors... network issues having nothing to do with your web host, over
utilization of resources by your host, server failure, or your web host
could even go out of business (it happens surprisingly often for
low-budget hosts).