Momma's explanation is clear !
I use PHPNet.fr (.org...), it's a french web hosting. For my wikis, I use shared server : no root acces, but 4GB of disk, one domain name included, really good services... for only 30 euros per year :)
2008/11/27 Momma mommahatesspam@gmail.com
Here's how I look at it:
SHARED SERVER (Cheapest): You buy one or more accounts from an ISP to host your domain(s). There may be 500 other user accounts on that same machine. You have no root access and have resource limits like total disk space, monthly bandwidth, number of databases, email accts, ftp accts, etc...
DEDICATED SERVER (most expensive): You lease an entire machine from a service provider. For an unmanaged system (what I have), the provider puts it online with the software you order and gives you the keys. You have complete root access to the machine and no one else has access it to unless you so permit. You are completely responsible for managing the server.
VIRTUAL SERVER (middle tier cost): Your website is very popular and you are either exceeding your disk or bandwidth allocation, or your ISP is going to kick you off because you are using too much CPU and hurting his other 499 users. A dedicated server is too expensive or complicated for you to manage -- or maybe more power than you need. The next viable option is a VIRTUAL SERVER. Instead of there being 500 indiv. user accounts on the machine, there may be 10 total virtual servers. To you, it appears that you have a dedicated server and have much of the control as if you owned the whole machine. You can host as many domains, or use as many resources, as your account allows. The actual root-type capabilities depends upon the virtual server software being used. I believe Virtuoso is a popular package. Find an ISP that provides virtual servers and see what system they provide and google for more info on capabilities.