You know what they are and have probably seen hundreds of them in various forms on websites, in software, and even cable and telephone service. But how many of them have you actually read? For most people Terms of Service (TOS) Agreements or Acceptable Use Policies are just legal formalities that can safely be ignored. When starting a website, particularly one involving business, it might be wise to take a closer look.
Typical TOS documents will include portions about the types of sites a web host will allow. For example, if you intend to host an adult site, it might behoove you to make sure the web hosting provider you are choosing will allow adult content. It has less to do with censorship than with security and excessive bandwidth and memory requirements of adult sites.
Other issues to consider are policies on mass email marketing (spamming), outage compensation, gaming, file sharing, and copyright. The last thing you want is to invest time and money into a project only to have the rug pulled out from under it by your web host. If you have any doubts, contact them ahead of time and confirm that your site will be permitted on their server, and always read the fine print.