Due to Overselling now web hosting prices are down.Overselling is basically selling more than what you can provide. In this case instead of putting 30 customers on a server, they put 300 customers. This is done based on the assumption that no user will use all of their allotted space and bandwidth.
This can only be taken to a certain extent before it starts becoming a problem, and the server starts getting overloaded. if everyone used all their bandwidth and disk space server will be down.
Overselling is a major problem of web hosting industry.