The user information list is located at the root of site collection/_catalogs/users/detail.aspx . When a site under the site collection is has added a user, or a list reference a user, that user is imported in the user information list. Please know that the user information list not the same as user table in the authentication system like active directory. There might be one thousand of user in the active directory, but there might be a 10 user in the user information list.