Redundancy and Failover these fancy words name very common sense concepts. When one computer (or part) fails, switch to another. Doing that seamlessly and quickly versus slowly with disruption defines one difference between good hosting and bad.
Unix vs Windows-Based Hosting. Whether a Unix-based site (usually Linux these days) is easier to administer than Windows based depends on your current skill set and the type of tools and level of access the web hosting company provides.
Managing Disk Space. Few things are less exciting than managing the disk space that always seems to be in too short a supply. But few things are more important to the health and well being of your site.
The Internet and How It Works In one sense, detailing the statement in the title would require at least a book. In another sense, it can't be fully explained at all, since there's no central authority that designs or implements the highly distributed entity called The Internet.