An ISP, or Internet Service Provider, provides dial-up or DSL (Digital Subscriber Line) connections for your home or office computers to the Internet. These connections are meant to be used for browser access only. The speed is just too slow for web server access, and the DNS requirement for web servers is just not part of the typical ISP offering.
A Web Hosting Company (or Web Presence Provider (WPP)) provides the other end of the Information Superhighway. It provides a server, and the high-speed connection to the Internet, but no dial-up or DSL service.
If you want to build and manage a web site, you will need both services. Often, the same company provides both services. However there is no reason you need to get both services from the same company. It is best to do as much comparison shopping as you can, and buy each service from the vendor that meets your needs best.
The webmaster and author of this Math Help site is Graeme McRae.