A shared web hosting service or virtual hosting service refers to a web site hosting service where many websites reside on one web server connected to the Internet. Each site has its own partition, or section/place on the server to keep it separate from other sites. This is generally the most economical/cheapest option for web hosting as many people share the overall cost of server maintenance.

The web hosting company will provide the server upgrade and maintenance, and web site owner will be focusing on web site content. 

Web hosting companies normally provide support for basic web hosting requirement, like web space, network bandwith, database, email service, ftp support, etc.  There may be some restrictions or limitations depending on which web hosting package you choose.

Besides, web hosting companies normally provide plenty of free add-on tools, which are easy to install, easy to use, easy to configure, like blog, forum, ecommerce, etc.  Which tools will be provided will be vary depending on web hosting package.

The following top web hosting comanies take big market share for web hosting businesses, and recommended here for the proven reliable and robust services:

GoDaddy - Hosting Plans starts from $4.25/month
1&1 - Host unlimited domains + get up to 5 FREE domains starts from $3.99/month
Yahoo! Web Hosting - Save 25% on your first 3-months of hosting service
DreamHost - Plans starting at $5.95/month
Blue Host - Free Domain, unlimited package with $6.95/month
Host Gator - Unlimited package at $7.95/month
Host Monster - Free domain, unlimited package at $3.95/month
IX Web Hosting - Free domain, unlimited packages from $3.95/month
IPowerWeb - Free domain, unlimited package at $5.95/month

Although the following companies are not as big as previous companies, they still provide pretty good service and deals:

Web Hosting Pad - Free domain, unlimited package at $2.99/month
Lunarpages - Free domain, unlimited package at $4.95/month
HostRocket - Unlimited package at $4.99/month
Just Host - Free domain, unlimited package at $3.95/month
PowerWeb Hosting - Free domain, unlimited package at $3.88/month
Omnis Network - Free domain, unlimited packge at $5.95/month
FatCow - Free domain, unlimited package at $5.50/month

More and more web hosting companies start to provide more reliable and robust solutions for business critical web sites now.  If you are interested in this, please check High Availability Web Hosting page.