Information regarding site locations is recorded on DNS tables or nameservers. These records are updated by Internet Presence & Service Providers to reflect the addition and changes of site locations. The update of these records is referred to as propagation. Propagation normally lasts 24-48 hours.
During this propagation period some ISPs will still point visitors to the web site located on your previous web host's servers while other ISPs, that have already updated their DNS Tables, will point your visitors to the web site located on your new web host's servers. The same is true for email that is sent to email@example.com. This is usually a frustrating and confusing time for the site owner and visitors as well. It is also not uncommon for a visitor to randomly see the new and old web sites each time they point their browser to your domain name due to the DNS Cache kept within their personal computers conflicting with the DNS entries provided by their ISP.