A web of trust is kind of a crowd sourcing platform that helps site users to see how trustworthy a website is based on the ratings of other users and also enables the user of the site to review the sites as well.
It is a crowd sourcing in the sense that it depends on the ratings of a group of reviewers with highest ratings on a website to determine how safe and trustworthy the site is or will be. it doesn't depend on the ratings a specific group of experts. The higher the ratings the safer the website is said to be.
