Here are more details on the features of PlayStation Plus: Games & Special Offers:
As a subscriber, you can expect to get your hands on free monthly PSN games, minis and PS One Classics from the PSN Store. You will be able to download these games during the month they are available until the new selection replaces them.
Once downloaded, the games are yours to keep and play as long as you are a member of PlayStation Plus. If your subscription lapses and then you re-subscribe, the games you have already downloaded will reactivate, so you still have access to them. Additionally, if you delete one of the titles from your PS3, but decide you want to get it back, then you can re-download from the PlayStation Plus section of the PSN Store if the game is still there, or, if the game has expired from the PlayStation Plus section of the PSN Store, you can simply search for it on PS Store and download it again at no extra cost.