- Sites that allow visitor interaction like posting comments without an account
- Sites that allow visitors to create accounts and interact with the site
- Sites that allow visitors to spend money on things like buying goods, buying services, or donating money.
Have a look at the sample privacy policies from this thread and see if any of the elements they refer to, you refer to your site.
Privacy is a huge area of activity. Check out the EFF site for informations of concern.
Coming to this late in the day, but the 3 definitions given by the first answer-ee really nail it in my opinion.
Rather than simply being an 'insurance policy' against legal/regulatory action, I think website owners should view this rather as an opportunity to show themselves as transparent and responsible. In a crowded market, this brings with it a competitive advantage.
In the UK, I would say every site should at least have a privacy/cookie usage policy, company number and contact details as a minimum standard of disclosure, whether you're capturing user data or not. Just like if you have a shop, you have an address, name of proprietor and a letterbox for correspondence.