It's payable if you watch any TV "as it is broadcast" so that means watching via a cable/satellite/freeview or watching stuff that is being streamed on iplayer the same time as on TV.

Weirdly though you don't need one to watch iplayer "catch up" content which is usually available about an hour after the initial showing.

So if you watch a lot of iplayer catch up you don't have to pay, but if you consume no BBC content and just use your TV to watch Sky1 or whatever then you do have to pay.