"Pay" channels are available from other companies in addition to the Freeview service. The core idea behind Freeview is that all the channels are free and there's only a one-off payment for the equipment and no contract.
Other companies offer extra content such as sport, movies, entertainment, children's programmes etc on a subscription or pay per view basis and you'll still receive the Freeview channels for free . These include Top Up TV and BT Vision which offer extra programming plus Sky Sports 1 & 2 and ESPN at extra cost.
You will need the right equipment to receive their channels and services. These companies are run separately to Freeview, which remains a not-for-profit organisation committed to providing free digital TV.