Those are two completely separate services.

Playstation Plus is the equivalent of Xbox Live Gold. If you're not familiar with any of those, then its basically a license that allows you to play Online games... With the added bonus of getting free games for your Playstation devices every month!

Playstation Now is a game streaming service, similar to Onlive or Nvidia's Grid. You can subscribe for a monthly fee or rent individual Playstation 3 games and you can play them on Playstation Now certified devices such as the Playstation 3 itself, PS4, PS Vita, Playstation TV, Sony Blu-Ray players or some Smart TVs (Sony and Samsung are supported, I believe)