As of today it seems that Playstation Now is available on the Playstation store in Belgium and the Netherlands, next to other countries in the US and Europe. With Playstation Now gamers can stream their classic Playstation 1, 2 and 3 games to another device like the Playstation 4. Games do not get installed on your Playstation 4, they are literally being streamed on the fly from the Playstation servers.
For people that have no clue or idea what Playstation Now is about there is also a trial package available, where you can test and play all the games for a full week. Over 100 titles are currently available in Playstation Now and the service should be available on other devices, besides the Playstation 4, very soon.
Next to the trial, the standard payment options include renting for 24 hours, 3 days or a week or 16,99 euros per month as a subscription to continue playing whatever, whenever, wherever you want.
More info can also be found on the official Playstation Now website.