Playstation Plus (usually abbreviated as PS+) is a paid subscription service that provides users with enhanced services that are in addition to the basic service that is available to all Playstation Network users. Users who sign up for this service are given access to exclusive demos, online cloud save game storage, discounts and betas, as well as access to free games that change monthly. Users can purchase subscriptions for either 3 months ($17.99) or 12 months ($49.99) at retailers or on the store.
There are Full Game Trials for PS+ users, which are basically the first 60 minutes of a full retail game, where players can earn Trophies and can continue their progress if they choose to purchase the full game. At E3 in 2012, the Instant Game Collection was announced, which is a collection of 12 free games that rotate periodically.
The games offered for free via PS+ are similar to rentals, in that, if the PS+ subscription runs out, then that user will not be able to play the games that were downloaded for free. This doesn't extend to those games that had PS+ discounts and were purchased with money, as those remain accessible to the user even if their subscription lapses.
