Most small websites on the ‘net sit on shared hosting of some kind or another 1. Shared hosting means that a small number of servers handle all the requests for a large number of web sites. How that’s achieved varies, but the bottom line is that it’s a shared infrastructure. It’s a bit like living in shared accommodation. There’s a single door through which everyone gets into the building, then a number of apartments which have their own doors. But they all share the same electricity supply and water and other utilities.
With shared web hosting, all the traffic comes into the same web host network and web server cluster, and is then handled by all the different web site configurations. In the same way that there are people who would like to break into your apartment, there are people who’d like to break into your web site to steal stuff, deface it, or to try and gain further access to the shared infrastructure.
- this is an educated guess [↩]