If you watch live TV programs on any online TV service, including ITVX, All 4, Amazon Prime Video, Now TV, or Sky Go, then you need to be covered by a TV Licence.
However, if you only watch on-demand programs on any TV service apart from BBC iPlayer, you don't need a TV Licence.
You also don't need a TV Licence to watch videos or clips on demand on YouTube, but you do need a TV Licence if you watch TV programs live on YouTube, such as Sky News live.
Any program that is part of a TV channel and broadcast or transmitted for everyone to watch at the same time requires a TV Licence.